Method for driving liquid crystal display device

ABSTRACT

In a period T in , p-th input image data (p is a positive integer) and (p+1)th input image data are input to a liquid crystal display device. In a period T, i-th original image data (i is a positive integer) and (i+1)th original image data are generated based on the input image data. J number of sub-images (J is an integer equal to or more than 3) are generated based on the i-th original image data. In the period T, the J number of sub-images are sequentially displayed. At least one of the i-th original image data and the (i+1)th original image data is in an intermediate state between the p-th input image data and the (p+1)th input image data. Each of the sub-images exhibits one of first brightness and second brightness. At least one sub-image among the J number of sub-images is different from the other sub-images.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to devices and driving methods thereof.Specifically, the present invention relates to semiconductor devices anddriving methods thereof. More specifically, the present inventionrelates to display devices and driving methods thereof, andparticularly, methods for improving quality of moving images by ahold-type driving method.

2. Description of the Related Art

In recent years, there has been growing interests in thin displaydevices. As substitutes for CRT displays, liquid crystal displays,plasma displays, projection displays, and the like have been developedand becoming popular. Further, field emission displays, inorganicelectroluminescence displays, organic electroluminescence displays,electronic paper, and the like have been developed as next-generationdisplay devices.

In a display portion provided in the above-described display device,pixels which are minimum units for forming an image are arranged. Eachof the pixels emits light with luminance in accordance with image data,and an image is displayed on the display portion.

When a moving image is displayed by such a display device, a pluralityof images are displayed quickly, several tens of times per second. Acycle in which a plurality of images are displayed (or a cycle in whichimage data is input to the display device) is referred to as one frameperiod.

Here, driving methods of display devices can be classified according totemporal distribution of luminance of a pixel in one frame period. In ahold-type driving method which is used mainly in an active matrixdisplay device, luminance of a pixel is constant in one frame period. Onthe other hand, in an impulse-type driving method which is used in CRTand the like, intense light is emitted once in one frame period andthereafter, luminance of a pixel immediately decreases and light is notemitted any more. In the impulse driving method, a non-lighting statedominates the most part of one frame period.

It has become obvious that hold-type display devices have the followingproblems of motion blur: in displaying moving images, a moving objectappears to have traces when a part of the image moves or the whole imageis blurred when the whole image moves. It is said that this is becausean image displayed on a hold-type display device is kept still duringone frame period, while a human predicts motion of the moving object andsees a direction in which the moving object is supposed to move. Thatis, this is because a discrepancy between movement of human eyes andmovement of the moving image. In an impulse-type display device, sincean image appears for a moment and disappears immediately, there is noproblem of such a discrepancy between human eyes and a moving image, andthus, a problem of motion blur does not occur.

Typically, two methods for solving the problem of motion blur in thehold-type display device are proposed (see Patent Document 1). The firstmethod is a method in which a period for displaying an original imageand a period for display a black image are provided in one frame period.By this method, display can be closer to pseudo impulse-type display,and fewer afterimages can be perceived; thus, quality of moving imagescan be improved (see Patent Documents 2 and 3). The second method is amethod in which one frame period is shortened (i.e., frame frequency isincreased). Accordingly, movement of an image can follow movement ofhuman eyes and movement of the image is smooth, which leads toimprovement of quality of moving images (see Patent Document 4).Further, as a technique for improving the first method, a method inwhich an image which is darker than an original image is displayedinstead of a black image to improve quality of moving images isdisclosed (see Patent Documents 5 and 6, and Non-patent Documents 1 to3). Further, a method in which a driving method is changed depending onconditions is disclosed (see Patent Documents 7 and 8).

Patent Document 1: Japanese Published Patent Application No. H4-302289

Patent Document 2: Japanese Published Patent Application No. H9-325715

Patent Document 3: Japanese Published Patent Application No. 2000-200063

Patent Document 4: Japanese Published Patent Application No. 2005-268912

Patent Document 5: Japanese Published Patent Application No. 2002-23707

Patent Document 6: Japanese Published Patent Application No. 2004-240317

Patent Document 7: Japanese Published Patent Application No. 2002-91400

Patent Document 8: Japanese Published Patent Application No. 2004-177575

Non-patent Document 1: “New Technologies for Large-Sized High-QualityLCD TV”, SID'05 DIGEST, 60.2, pp. 1734-1737 (2005)

Non-patent Document 2: “Amorphous Silicon Based 40” LCD TV Using UltraFast OCB Mode”, SID'06 DIGEST, 69.4, pp. 1950-1953 (2006)

Non-patent Document 3: “Progress of IPS-Pro Technology for LCD-TVs”,SID'06 DIGEST, 69.5, pp. 1954-1957 (2006)

SUMMARY OF THE INVENTION

As described above, a variety of methods have been studied to solve theproblem of motion blur in hold-type display devices. However, suchmethods do not provide sufficient effects and have not yet have qualityof moving images equal to that of impulse-type display devices. Further,in the method in which display of a black image is made closer to pseudoimpulse-type display, flickers are increased. When a black image isdisplayed, luminance of an image is decreased, and power consumption isincreased to provide the same level of luminance as in the case where ablack image is not inserted. In the method of increasing framefrequency, data processing is complicated, and a driver circuit forhigh-speed processing is needed, which causes problems such as increasein manufacturing cost, heat generation due to the data processing, andincrease in power consumption.

In view of the foregoing problems, a main object of the invention is toprovide a display device in which quality (image quality) of stillimages and moving images is improved and a driving method of the displaydevice. In particular, objects are to provide a display device withoutthe problem of motion blur and a driving method of the display device,to provide a display device without flickers and a driving method of thedisplay device, to provide a display device with wider viewing anglesand a driving method of the display device, to provide a display devicewith higher response speed and a driving method of the display device,to provide a display device with reduced power consumption and a drivingmethod of the display device, to provide a display device with reducedmanufacturing cost and a driving method of the display device, and thelike.

In one aspect of a method for driving a liquid crystal display device ofthe invention, p-th input image data (p is a positive integer) and(p+1)th input image data are input to the liquid crystal display devicein a period T_(in); in a period T, i-th original image data (i is apositive integer) and (i+1)th original image data are generated based onthe input image data; J number of sub-images (J is an integer equal toor more than 3) are generated based on the i-th original image data; inthe period T, the J number of sub-images are sequentially displayed; atleast one of the i-th original image data and the (i+1)th original imagedata is in an intermediate state between the p-th input image data andthe (p+1)th input image data; each of the sub-images exhibits one offirst brightness and second brightness; and at least one sub-image amongthe J number of sub-images is different from the other sub-images.

There are the following advantages when at least one sub-image among theJ number of sub-images is different from the other sub-images. Forexample, when different sub-images are sequentially displayed in theperiod T, at least one pixel whose brightness is temporally changedexists in the period T, In other words, impulse driving is performed inthe pixel. That is, when at least one sub-image among the J number ofsub-images is different from the other sub-images, impulse-type displaycan be performed; thus, quality of moving images can be improved.Further, the frequency of writing to each pixel in the period T isincreased as J is increased; thus, lack of writing voltage to the pixelby constant charge driving is solved, and response time of a displayelement can be reduced.

In another aspect of a method for driving a liquid crystal displaydevice of the invention, at least one of the J number of sub-images is ablack image in the above-described structure.

In another aspect of a method for driving a liquid crystal displaydevice of the invention, p-th input image data (p is a positive integer)and (p+1)th input image data are input to the liquid crystal displaydevice in a period T_(in); in a period T, i-th original image data (i isa positive integer) and (i+1)th original image data are generated basedon the input image data; J number of sub-images (J is an integer equalto or more than 3) are generated based on the i-th original image data;in the period T, the J number of sub-images are sequentially displayed;at least one of the i-th original image data and the (i+1)th originalimage data is in an intermediate state between the p-th input image dataand the (p+1)th input image data; each of the sub-images exhibits one offirst brightness and second brightness; and all of the J number ofsub-images are the same images.

In another aspect of a method for driving a liquid crystal displaydevice of the invention, p-th input image data (p is a positive integer)and (p+1)th input image data are input to the liquid crystal displaydevice in a period T_(in); in a period T, i-th original image data (i isa positive integer) and (i+1)th original image data are generated basedon the input image data; a first sub-image and a second sub-image aregenerated based on the i-th original image data; in the period T, thefirst sub-image and the second sub-image are sequentially displayed; atleast one of the i-th original image data and the (i+1)th original imagedata is in an intermediate state between the p-th input image data andthe (p+1)th input image data; the period T_(in) is more than twice theperiod T; the first sub-image and the second sub-image are differentfrom each other; and one of the first sub-image and the second sub-imageis a black image.

In another aspect of a method for driving a liquid crystal displaydevice of the invention, p-th input image data (p is a positive integer)and (p+1)th input image data are input to the liquid crystal displaydevice in a period T_(in); in a period T, i-th original image data (i isa positive integer) and (i+1)th original image data are generated basedon the input image data; a first sub-image and a second sub-image aregenerated based on the i-th original image data; in the period T, thefirst sub-image and the second sub-image are sequentially displayed; atleast one of the i-th original image data and the (i+1)th original imagedata is in an intermediate state between the p-th input image data andthe (p+1)th input image data; the period T_(in) is more than twice theperiod T; and the first sub-image and the second sub-image are the sameimages.

Note that various types of switches, for example, an electrical switchand a mechanical switch can be used. That is, any element can be usedwithout being limited to a particular type as long as it can control acurrent flow. For example, a transistor (e.g., a bipolar transistor or aMOS transistor), a diode (e.g., a PN diode, a PIN diode, a Schottkydiode, a metal-insulator-metal (MIM) diode, ametal-insulator-semiconductor (MIS) diode, or a diode-connectedtransistor), a thyristor, or the like can be used as a switch.Alternatively, a logic circuit in which such elements are combined canbe used as a switch.

Examples of a mechanical switch include a switch formed using a microelectro mechanical system (MEMS) technology, such as a digitalmicromirror device (DMD). Such a switch includes an electrode which canbe moved mechanically, and operates by controlling connection ornon-connection based on movement of the electrode.

When a transistor is used as a switch, polarity (a conductivity type) ofthe transistor is not particularly limited since it operates just as aswitch. Note that when off-current is preferably to be suppressed, atransistor of polarity with smaller off-current is preferably used.Examples of a transistor with smaller off-current include a transistorhaving an LDD region and a transistor having a multi-gate structure.Further, an n-channel transistor is preferably used when a transistoroperates with a potential of a source terminal closer to a potential ofa low potential side power supply (e.g., VS, GND, or 0 V). On the otherhand, a p-channel transistor is preferably used when a transistoroperates with a potential of a source terminal closer to a potential ofa high potential side power supply (e.g., V_(dd)). This is because whenthe n-channel transistor operates with the potential of the sourceterminal closer to the low potential side power supply and when thep-channel transistor operates with the potential of the source terminalcloser to the high potential side power supply, an absolute value ofgate-source voltage can be increased; thus, the transistor can moreprecisely operate as a switch. Moreover, this is because reduction inoutput voltage does not occur often because the transistor does notoften perform a source follower operation.

Note that a CMOS switch may also be employed by using both n-channel andp-channel transistors. A CMOS switch can easily function as a switchsince current can flow when one of the n-channel transistor and thep-channel transistor is turned on. For example, voltage can be output asappropriate whether voltage of an input signal to the switch is high orlow. Further, since a voltage amplitude value of a signal for turningon/off the switch can be decreased, power consumption can be reduced.

Note that when a transistor is used as a switch, the switch includes aninput terminal (one of a source terminal and a drain terminal), anoutput terminal (the other of the source terminal and the drainterminal), and a terminal (a gate terminal) for controlling electricalconduction. On the other hand, when a diode is used as a switch, theswitch does not have a terminal for controlling electrical conduction insome cases. Therefore, when a diode is used as a switch, the number ofwirings for controlling terminals can be reduced compared with the casewhere a transistor is used as a switch.

Note that when it is explicitly described that A and B are connected,the case where A and B are electrically connected, the case where A andB are functionally connected, and the case where A and B are directlyconnected are included. Here, each of A and B is an object (e.g., adevice, an element, a circuit, a wiring, an electrode, a terminal, aconductive film, or a layer). Accordingly, another element may beprovided in a connection relationship shown in drawings and texts,without being limited to a predetermined connection relationship, forexample, connection relationships shown in the drawings and the texts.

For example, when A and B are electrically connected, one or moreelements which enable electrical connection of A and B (e.g., a switch,a transistor, a capacitor, an inductor, a resistor, or a diode) may beprovided between A and B. Alternatively, when A and B are functionallyconnected, one or more circuits which enable functional connection of Aand B (e.g., a logic circuit such as an inverter, a NAND circuit, or aNOR circuit; a signal converter circuit such as a DA converter circuit,an AD converter circuit, or a gamma correction circuit; a potentiallevel converter circuit such as a power supply circuit (e.g., a boostercircuit or a voltage step-down circuit) or a level shifter circuit forchanging potential level of a signal; a voltage source; a currentsource; a switching circuit; or an amplifier circuit which can increasesignal amplitude, the amount of current, or the like, such as anoperational amplifier, a differential amplifier circuit, a sourcefollower circuit, or a buffer circuit; a signal generation circuit; amemory circuit; or a control circuit may be provided between A and B.Alternatively, when A and B are directly connected, A and B may bedirectly connected without interposing another element or anothercircuit therebetween.

When it is explicitly described that A and B are directly connected, thecase where A and B are directly connected (i.e., the case where A and Bare connected without interposing another element or another circuittherebetween) and the case where A and B are electrically connected(i.e., the case where A and B are connected by interposing anotherelement or another circuit therebetween) are included.

In addition, when it is explicitly described that A and B areelectrically connected, the case where A and B are electricallyconnected (i.e., the case where A and B are connected by interposinganother element or another circuit therebetween), the case where A and Bare functionally connected (i.e., the case where A and B arefunctionally connected by interposing another circuit therebetween), andthe case where A and B are directly connected (i.e., the case where Aand B are connected without interposing another element or anothercircuit therebetween) are included. That is, when it is explicitlydescribed that A and B are electrically connected, the description isthe same as the case where it is explicitly described only that A and Bare connected.

Note that a display element, a display device which is a deviceincluding a display element, a light-emitting element, and alight-emitting device which is a device including a light-emittingelement can employ various modes and can include various elements. Forexample, as a display element, a display device, a light-emittingelement, and a light-emitting device, a display medium whose contrast,luminance, reflectivity, transmittance, or the like is changed byelectromagnetic action, such as an EL (electroluminescence) element(e.g., an EL element including both organic and inorganic materials, anorganic EL element, or an inorganic EL element), an electron emitter, aliquid crystal element, electronic ink, an electrophoretic element, agrating light valve (GLV), a plasma display panel (PDP), a digitalmicromirror device (DMD), a piezoelectric ceramic display, or a carbonnanotube can be used. Note that display devices using an EL elementinclude an EL display in its category; display devices using an electronemitter include a field emission display (FED) and an SED-type flatpanel display (SED: surface-conduction electron-emitter display) in itscategory; display devices using a liquid crystal element include aliquid crystal display (e.g., a transmissive liquid crystal display, atransflective liquid crystal display, a reflective liquid crystaldisplay, a direct-view liquid crystal display, or a projection typeliquid crystal display) in its category; and display devices usingelectronic ink include electronic paper in its category.

Note that an EL element is an element including an anode, a cathode, andan EL layer interposed between the anode and the cathode. Examples ofthe EL layer include various types of EL layers, for example, a layerutilizing light emission (fluorescence) from a singlet exciton, a layerutilizing light emission (phosphorescence) from a triplet exciton, alayer utilizing light emission (fluorescence) from a singlet exciton andlight emission (phosphorescence) from a triplet exciton, a layer formedof an organic material, a layer formed of an inorganic material, a layerformed of an organic material and an inorganic material, a layerincluding a high molecular material, a layer including a low molecularmaterial, and a layer including a high molecular material and a lowmolecular material. Note that the invention is not limited thereto, andvarious types of EL elements can be used.

Note that an electron emitter is an element in which electrons areextracted by high electric field concentration on a pointed cathode. Forexample, the electron emitter may be any one of a Spindt type, a carbonnanotube (CNT) type, a metal-insulator-metal (MIM) type in which ametal, an insulator, and a metal are stacked, ametal-insulator-semiconductor (MIS) type in which a metal, an insulator,and a semiconductor are stacked, a MOS type, a silicon type, a thin-filmdiode type, a diamond type, a surface conduction emitter SCD type, athin film type in which a metal, an insulator, a semiconductor, and ametal are stacked, a HEED type, an EL type, a porous silicon type, asurface-conduction electron-emitter (SED) type, and the like. However,the invention is not limited thereto, and various elements can be usedas an electron emitter.

Note that a liquid crystal element is an element which controlstransmission or non-transmission of light by optical modulation actionof a liquid crystal and includes a pair of electrodes and a liquidcrystal. Optical modulation action of the liquid crystal is controlledby an electric filed applied to the liquid crystal (including ahorizontal electric field, a vertical electric field, and an obliqueelectric field). The following liquid crystals can be used for a liquidcrystal element: a nematic liquid crystal, a cholesteric liquid crystal,a smectic liquid crystal, a discotic liquid crystal, a thermotropicliquid crystal, a lyotropic liquid crystal, a low molecular liquidcrystal, a high molecular liquid crystal, a ferroelectric liquidcrystal, an anti-ferroelectric liquid crystal, a main chain type liquidcrystal, a side chain type polymer liquid crystal, a plasma addressedliquid crystal (PALC), a banana-shaped liquid crystal, a TN (twistednematic) mode, an STN (super twisted nematic) mode, an IPS(in-plane-switching) mode, an FFS (fringe field switching) mode, an MVA(multi-domain vertical alignment) mode, a PVA (patterned verticalalignment) mode, an ASV (advanced super view) mode, an ASM (axiallysymmetric aligned microcell) mode, an OCB (optical compensatedbirefringence) mode, an ECB (electrically controlled birefringence)mode, an FLC (ferroelectric liquid crystal) mode, an AFLC(anti-ferroelectric liquid crystal) mode, a PDLC (polymer dispersedliquid crystal) mode, and a guest-host mode. Note that the invention isnot limited thereto, and various kinds of liquid crystal elements can beused.

Note that examples of electronic paper include a device displaying animage by molecules which utilizes optical anisotropy, dye molecularorientation, or the like; a device displaying an image by particleswhich utilizes electrophoresis, particle movement, particle rotation,phase change, or the like; a device displaying an image by moving oneend of a film; a device using coloring properties or phase change ofmolecules; a device using optical absorption by molecules; and a deviceusing self-light emission by combination of electrons and holes. Forexample, the followings can be used for electronic paper: microcapsuletype electrophoresis, horizontal type electrophoresis, vertical typeelectrophoresis, a spherical twisting ball, a magnetic twisting ball, acolumn twisting ball, a charged toner, an electro liquid powder,magnetic electrophoresis, a magnetic heat-sensitive type element, anelectrowetting type element, a light-scattering (transparent-opaquechange) type element, a cholesteric liquid crystal and a photoconductivelayer, a cholesteric liquid crystal, a bistable nematic liquid crystal,a ferroelectric liquid crystal, a liquid crystal dispersed type elementwith a dichroic dye, a movable film, coloring and decoloring propertiesof a leuco dye, photochromism, electrochromism, electrodeposition,flexible organic EL, and the like. Note that the invention is notlimited thereto, and various types of electronic paper can be used. Byusing a microcapsule electrophoretic device, defects of theelectrophoresis type, which are aggregation and precipitation ofphoresis particles, can be solved. Electro liquid powder has advantagessuch as high-speed response, high reflectivity, wide viewing angle, lowpower consumption, and memory properties.

A plasma display includes a substrate having a surface provided with anelectrode, and a substrate having a surface provided with an electrodeand a minute groove in which a phosphor layer is formed. In the plasmadisplay, the substrates are opposite to each other with a narrowinterval, and a rare gas is sealed therein. Display can be performed byapplying voltage between the electrodes to generate an ultraviolet rayso that the phosphor emits light. Note that the plasma display panel maybe a DC type PDP or an AC type PDP. As a driving method of the plasmadisplay panel, ASW (address while sustain) driving, ADS (address displayseparated) driving in which a subframe is divided into a reset period,an address period, and a sustain period, CLEAR (high-contrast, lowenergy address and reduction of false contour sequence) driving, ALIS(alternate lighting of surfaces) method, TERES (technology of reciprocalsustainer) driving, and the like can be used. Note that the invention isnot limited thereto, and various types of plasma displays can be used.

Note that electroluminescence, a cold cathode fluorescent lamp, a hotcathode fluorescent lamp, an LED, a laser light source, a mercury lamp,or the like can be used for a light source needed for a display device,such as a liquid crystal display device (a transmissive liquid crystaldisplay, a transflective liquid crystal display, a reflective liquidcrystal display, a direct-view liquid crystal display, and a projectiontype liquid crystal display), a display device using a grating lightvalve (GLV), and a display device using a digital micromirror device(DMD). Note that the invention is not limited thereto, and various lightsources can be used.

Note that a structure of a transistor can employ various modes. Forexample, a multi-gate structure having two or more gate electrodes maybe employed. When the multi-gate structure is employed, a structurewhere a plurality of transistors are connected in series is providedsince channel regions are connected in series. The multi-gate structurerealizes reduction in off-current and improvement in reliability due toimprovement in withstand voltage of the transistor. Further, byemploying the multi-gate structure, drain-source current does not changemuch even if drain-source voltage changes when the transistor operatesin a saturation region; thus, the slope of voltage-currentcharacteristics can be flat. By utilizing the characteristics in whichthe slope of the voltage-current characteristics is flat, an idealcurrent source circuit and an active load having an extremely highresistance value can be realized. Thus, a differential circuit or acurrent mirror circuit having excellent properties can be realized. Asanother example, a structure where gate electrodes are formed above andbelow a channel may be employed. By employing the structure where gateelectrodes are formed above and below the channel, a channel region isenlarged; thus, a subthreshold swing can be reduced because the amountof current is increased or a depletion layer is easily formed. When thegate electrodes are formed above and below the channel, it seems that aplurality of transistors are connected in parallel.

Alternatively, a structure where a gate electrode is formed above achannel region, a structure where a gate electrode is formed below achannel region, a staggered structure, or an inversely staggeredstructure may be employed. Further, a structure where a channel regionis divided into a plurality of regions, or a structure where a pluralityof channel regions are connected in parallel or in series may beemployed. Moreover, a structure where a source electrode or a drainelectrode overlaps with a channel region (or part thereof) may beemployed. By employing the structure where the source electrode or thedrain electrode overlaps with the channel region (or part thereof), anunstable operation due to accumulation of charge in part of the channelregion can be prevented. Alternatively, an LDD region may be provided.By providing the LDD region, off-current can be reduced, or reliabilitycan be improved by improvement in withstand voltage of the transistor.Further, by providing the LDD region, drain-source current does notchange much even if drain-source voltage changes when the transistoroperates in the saturation region, so that characteristics where a slopeof voltage-current characteristics is flat can be obtained.

Note that various types of transistors can be used, and the transistorcan be formed using various types of substrates. Accordingly, all ofcircuits which are necessary to realize a predetermined function can beformed using the same substrate. For example, all of the circuits whichare necessary to realize the predetermined function can be formed usingvarious substrates such as a glass substrate, a plastic substrate, asingle crystalline substrate, or an SOI substrate. When all of thecircuits which are necessary to realize the predetermined function areformed using the same substrate, cost can be reduced by reduction in thenumber of component parts or reliability can be improved by reduction inthe number of connections between circuit components. Alternatively,part of the circuits which is necessary to realize the predeterminedfunction may be formed using one substrate and another part of thecircuits which is necessary to realize the predetermined function may beformed using another substrate. That is, not all of the circuits whichare necessary to realize the predetermined function are required to beformed using the same substrate. For example, part of the circuits whichis necessary to realize the predetermined function may be formed over aglass substrate by using transistors and another part of the circuitswhich is necessary to realize the predetermined function may be formedusing a single crystalline substrate, and an IC chip formed by atransistor using the single crystalline substrate may be connected tothe glass substrate by COG (chip on glass) so that the IC chip isprovided over the glass substrate. Alternatively, the IC chip may beconnected to the glass substrate by TAB (tape automated bonding) or aprinted wiring board. When part of the circuits is formed using the samesubstrate in such a manner, cost can be reduced by reduction in thenumber of component parts or reliability can be improved by reduction inthe number of connections between circuit components. In addition,circuits in a portion with high driving voltage or a portion with highdriving frequency consume large power. Accordingly, the circuits in suchportions are formed using a single crystalline substrate, for example,instead of using the same substrate, and an IC chip formed by thecircuit is used; thus, increase in power consumption can be prevented.

Note that one pixel corresponds to one element whose brightness can becontrolled. For example, one pixel corresponds to one color element, andbrightness is expressed with one color element. Accordingly, in the caseof a color display device having color elements of R (red), G (green),and B (blue), the smallest unit of an image is formed of three pixels ofan R pixel, a G pixel, and a B pixel. Note that the color elements arenot limited to three colors, and color elements of more than threecolors may be used and/or a color other than RGB may be used. Forexample, RGBW can be employed by adding W (white). Alternatively, RGBadded with one or more colors of yellow, cyan, magenta, emerald green,vermilion, and the like may be used. Further alternatively, a colorsimilar to at least one of R, G, and B may be added to RGB. For example,R, G, B1, and B2 may be used. Although both B1 and B2 are blue, theyhave slightly different frequencies. Similarly, R1, R2, G and B may beused. By using such color elements, display which is closer to a realobject can be performed, and power consumption can be reduced. Asanother example, when brightness of one color element is controlled byusing a plurality of regions, one region may correspond to one pixel.For example, when area ratio gray scale display is performed or asubpixel is included, a plurality of regions which control brightnessare provided in one color element and gray scales are expressed with allof the regions, and one region which controls brightness may correspondto one pixel. In that case, one color element is formed of a pluralityof pixels. Alternatively, even when a plurality of the regions whichcontrol brightness are provided in one color element, these regions maybe collected and one color element may be referred to as one pixel. Inthat case, one color element is formed of one pixel. In addition, whenbrightness of one color element is controlled by a plurality of regions,regions which contribute to display may have different area dimensionsdepending on pixels in some cases. Alternatively, in a plurality of theregions which control brightness in one color element, signals suppliedto respective regions may slightly vary to widen a viewing angle. Thatis, potentials of pixel electrodes included in the plurality of theregions in one color element may be different from each other.Accordingly, voltages applied to liquid crystal molecules vary dependingon the pixel electrodes. Thus, the viewing angle can be widened.

Note that when it is explicitly described as one pixel (for threecolors), it corresponds to the case where three pixels of R, G and B areconsidered as one pixel. When it is explicitly described as one pixel(for one color), it corresponds to the case where a plurality of theregions provided in each color element are collectively considered asone pixel.

Note that pixels are provided (arranged) in matrix in some cases. Here,description that pixels are provided (arranged) in matrix includes thecase where the pixels are arranged in a straight line or in a jaggedline in a longitudinal direction or a lateral direction. For example,when full-color display is performed with three color elements (e.g.,RGB), the following cases are included therein: the case where thepixels are arranged in stripes, the case where dots of the three colorelements are arranged in a delta pattern, and the case where dots of thethree color elements are provided in Bayer arrangement. Note that thecolor elements are not limited to three colors, and color elements ofmore than three colors may be employed, for example, RGBW (W correspondsto white) or RGB added with one or more of yellow, cyan, magenta, andthe like. In addition, the size of display regions may vary inrespective dots of color elements. Thus, power consumption can bereduced or the life of a display element can be prolonged.

Note that an active matrix method in which an active element is includedin a pixel or a passive matrix method in which an active element is notincluded in a pixel can be used.

In the active matrix method, as an active element (a non-linearelement), various active elements (non-linear elements), such as ametal-insulator-metal (MIM) and a thin film diode (TFD) can be used aswell as a transistor. Since such an element has a small number ofmanufacturing steps, manufacturing cost can be reduced or the yield canbe improved. Further, since the size of the element is small, anaperture ratio can be increased, and reduction in power consumption andhigh luminance can be achieved.

As a method other than the active matrix method, the passive matrixmethod in which an active element (a non-linear element) is not used canalso be used. Since an active element (a non-linear element) is notused, the number of manufacturing steps is small, so that manufacturingcost can be reduced or the yield can be improved. Further, since anactive element (a non-linear element) is not used, an aperture ratio canbe increased, and reduction in power consumption and high luminance canbe achieved.

Note that a transistor is an element having at least three terminals ofa gate, a drain, and a source. The transistor includes a channel regionbetween a drain region and a source region, and current can flow throughthe drain region, the channel region, and the source region. Here, sincethe source and the drain of the transistor may change depending on astructure, operating conditions, and the like of the transistor, it isdifficult to define which is a source or a drain. Therefore, in thisdocument (the specification, the claims, the drawings, and the like), aregion functioning as a source and a drain is not called the source orthe drain in some cases. In such a case, one of the source and the drainmay be referred to as a first terminal and the other thereof may bereferred to as a second terminal, for example. Alternatively, one of thesource and the drain may be referred to as a first electrode and theother thereof may be referred to as a second electrode. Furtheralternatively, one of the source and the drain may be referred to as asource region and the other thereof may be referred to as a drainregion.

In addition, a transistor may be an element having at least threeterminals of a base, an emitter, and a collector. In this case also, oneof the emitter and the collector may be referred to as a first terminaland the other terminal may be referred to as a second terminal.

Note that a gate corresponds to all or part of a gate electrode and agate wiring (also referred to as a gate line, a gate signal line, a scanline, a scan signal line, or the like). A gate electrode corresponds topart of a conductive film which overlaps with a semiconductor forming achannel region with a gate insulating film interposed therebetween. Notethat in some cases, part of the gate electrode overlaps with an LDD(lightly doped drain) region or a source region (or a drain region) withthe gate insulating film interposed therebetween. A gate wiringcorresponds to a wiring for connecting gate electrodes of transistors, awiring for connecting gate electrodes included in pixels, or a wiringfor connecting a gate electrode to another wiring.

However, there is a portion (a region, a conductive film, a wiring, orthe like) which functions as both a gate electrode and a gate wiring.Such a portion (a region, a conductive film, a wiring, or the like) maybe called either a gate electrode or a gate wiring. That is, there is aregion where a gate electrode and a gate wiring cannot be clearlydistinguished from each other. For example, when a channel regionoverlaps with part of an extended gate wiring, the overlapped portion(region, conductive film, wiring, or the like) functions as both a gatewiring and a gate electrode. Accordingly, such a portion (a region, aconductive film, a wiring, or the like) may be called either a gateelectrode or a gate wiring.

Note that a portion (a region, a conductive film, a wiring, or the like)which is formed of the same material as a gate electrode and forms thesame island as the gate electrode to be connected to the gate electrodemay also be called a gate electrode. Similarly, a portion (a region, aconductive film, a wiring, or the like) which is formed of the samematerial as a gate wiring and forms the same island as the gate wiringto be connected to the gate wiring may also be called a gate wiring. Ina strict sense, such a portion (a region, a conductive film, a wiring,or the like) does not overlap with a channel region or does not have afunction of connecting the gate electrode to another gate electrode insome cases. However, there is a portion (a region, a conductive film, awiring, or the like) which is formed of the same material as a gateelectrode or a gate wiring and forms the same island as the gateelectrode or the gate wiring to be connected to the gate electrode orthe gate wiring in relation to a specification in manufacturing and thelike. Thus, such a portion (a region, a conductive film, a wiring, orthe like) may also be called either a gate electrode or a gate wiring.

In a multi-gate transistor, for example, a gate electrode is oftenconnected to another gate electrode by using a conductive film which isformed of the same material as the gate electrode. Since such a portion(a region, a conductive film, a wiring, or the like) is for connectingthe gate electrode and another gate electrode, it may be called a gatewiring, and it may also be called a gate electrode since a multi-gatetransistor can be considered as one transistor. That is, a portion (aregion, a conductive film, a wiring, or the like) which is formed of thesame material as a gate electrode or a gate wiring and forms the sameisland as the gate electrode or the gate wiring to be connected to thegate electrode or the gate wiring may be called either a gate electrodeor a gate wiring. In addition, part of a conductive film which connectsthe gate electrode and the gate wiring and is formed of a materialdifferent from that of the gate electrode or the gate wiring may also becalled either a gate electrode or a gate wiring.

Note that a gate terminal corresponds to part of a portion (a region, aconductive film, a wiring, or the like) of a gate electrode or a portion(a region, a conductive film, a wiring, or the like) which iselectrically connected to the gate electrode.

When a wiring is called a gate wiring, a gate line, a gate signal line,a scan line, a scan signal line, or the like, there is the case where agate of a transistor is not connected to the wiring. In this case, thegate wiring, the gate line, the gate signal line, the scan line, or thescan signal line corresponds to a wiring formed in the same layer as thegate of the transistor, a wiring formed of the same material as the gateof the transistor, or a wiring formed at the same time as the gate ofthe transistor in some cases. Examples of such a wiring include a wiringfor storage capacitance, a power supply line, and a reference potentialsupply line.

A source corresponds to all or part of a source region, a sourceelectrode, and a source wiring (also referred to as a source line, asource signal line, a data line, a data signal line, or the like). Asource region corresponds to a semiconductor region containing a largeamount of p-type impurities (e.g., boron or gallium) or n-typeimpurities (e.g., phosphorus or arsenic). Accordingly, a regioncontaining a small amount of p-type impurities or n-type impurities, aso-called LDD (lightly doped drain) region is not included in the sourceregion. A source electrode is part of a conductive layer formed of amaterial different from that of a source region and electricallyconnected to the source region. However, there is the case where asource electrode and a source region are collectively called a sourceelectrode. A source wiring corresponds to a wiring for connecting sourceelectrodes of transistors, a wiring for connecting source electrodesincluded in pixels, or a wiring for connecting a source electrode toanother wiring.

However, there is a portion (a region, a conductive film, a wiring, orthe like) functioning as both a source electrode and a source wiring.Such a portion (a region, a conductive film, a wiring, or the like) maybe called either a source electrode or a source wiring. That is, thereis a region where a source electrode and a source wiring cannot beclearly distinguished from each other. For example, when a source regionoverlaps with part of an extended source wiring, the overlapped portion(region, conductive film, wiring, or the like) functions as both asource wiring and a source electrode. Accordingly, such a portion (aregion, a conductive film, a wiring, or the like) may be called either asource electrode or a source wiring.

Note that a portion (a region, a conductive film, a wiring, or the like)which is formed of the same material as a source electrode and forms thesame island as the source electrode to be connected to the sourceelectrode, or a portion (a region, a conductive film, a wiring, or thelike) which connects a source electrode and another source electrode mayalso be called a source electrode. Further, a portion which overlapswith a source region may be called a source electrode. Similarly, aregion which is formed of the same material as a source wiring and formsthe same island as the source wiring to be connected to the sourcewiring may also be called a source wiring. In a strict sense, such aportion (a region, a conductive film, a wiring, or the like) does nothave a function of connecting the source electrode to another sourceelectrode in some cases. However, there is a portion (a region, aconductive film, a wiring, or the like) which is formed of the samematerial as a source electrode or a source wiring and forms the sameisland as the source electrode or the source wiring to be connected tothe source electrode or the source wiring in relation to a specificationin manufacturing and the like. Thus, such a portion (a region, aconductive film, a wiring, or the like) may also be called either asource electrode or a source wiring.

For example, part of a conductive film which connects a source electrodeand a source wiring and is formed of a material which is different fromthat of the source electrode or the source wiring may be called either asource electrode or a source wiring.

Note that a source terminal corresponds to part of a source region, asource electrode, or a portion (a region, a conductive film, a wiring,or the like) which is electrically connected to the source electrode.

When a wiring is called a source wiring, a source line, a source signalline, a data line, a data signal line, or the like, there is the casewhere a source (a drain) of a transistor is not connected to the wiring.In this case, the source wiring, the source line, the source signalline, the data line, or the data signal line corresponds to a wiringformed in the same layer as the source (the drain) of the transistor, awiring formed of the same material as the source (the drain) of thetransistor, or a wiring formed at the same time as the source (thedrain) of the transistor in some cases. Examples of such a wiringinclude a wiring for storage capacitance, a power supply line, and areference potential supply line.

Note that a drain is similar to the source.

Note that a semiconductor device corresponds to a device having acircuit including a semiconductor element (e.g., a transistor, a diode,or a thyristor). The semiconductor device may also refer to all deviceswhich can function by utilizing semiconductor characteristics.Alternatively, the semiconductor device refers to a device including asemiconductor material.

A display element corresponds to an optical modulation element, a liquidcrystal element, a light-emitting element, an EL element (an organic ELelement, an inorganic EL element, or an EL element including bothorganic and inorganic materials), an electron emitter, anelectrophoresis element, a discharging element, a light-reflectingelement, a light diffraction element, a digital micromirror device(DMD), or the like. Note that the present invention is not limitedthereto.

A display device corresponds to a device including a display element.The display device may include a plurality of pixels having a displayelement. The display device may include a peripheral driver circuit fordriving a plurality of pixels. The peripheral driver circuit for drivinga plurality of pixels may be formed over the same substrate as theplurality of pixels. The display device may also include a peripheraldriver circuit provided over a substrate by wire bonding or bumpbonding, that is, an IC chip connected by so-called chip on glass (COG),TAB, or the like. Further, the display device may also include aflexible printed circuit (FPC) to which an IC chip, a resistor, acapacitor, an inductor, a transistor, or the like is attached. Thedisplay device may also include a printed wiring board (PWB) which isconnected through a flexible printed circuit (FPC) and the like and towhich an IC chip, a resistor, a capacitor, an inductor, a transistor, orthe like is attached. The display device may also include an opticalsheet such as a polarizing plate or a retardation plate. The displaydevice may also include a lighting device, a housing, an audio input andoutput device, an optical sensor, or the like. Here, a lighting devicesuch as a backlight unit may include a light guide plate, a prism sheet,a diffusion sheet, a reflective sheet, a light source (e.g., an LED or acold cathode fluorescent lamp), a cooling device (e.g., a water coolingtype or an air cooling type), or the like.

A lighting device corresponds to a device including a backlight unit, alight guide plate, a prism sheet, a diffusion sheet, a reflective sheet,a light source (e.g., an LED, a cold cathode fluorescent lamp, or a hotcathode fluorescent lamp), a cooling device, or the like.

A light-emitting device corresponds to a device including alight-emitting element or the like. A light-emitting device including alight-emitting element as a display element is a specific example of adisplay device.

A reflective device corresponds to a device including a light-reflectingelement, a light diffraction element, a light reflecting electrode, orthe like.

A liquid crystal display device corresponds to a display deviceincluding a liquid crystal element. Liquid crystal display devicesinclude a direct-view liquid crystal display, a projection liquidcrystal display, a transmissive liquid crystal display, a reflectiveliquid crystal display, a transflective liquid crystal display, and thelike in its category.

A driving device corresponds to a device including a semiconductorelement, an electric circuit, or an electronic circuit. Examples of thedriving device include a transistor (also referred to as a selectiontransistor, a switching transistor, or the like) which controls input ofa signal from a source signal line to a pixel, a transistor whichsupplies voltage or current to a pixel electrode, and a transistor whichsupplies voltage or current to a light-emitting element. Moreover,examples of the driving device include a circuit (also referred to as agate driver, a gate line driver circuit, or the like) which supplies asignal to a gate signal line, and a circuit (also referred to as asource driver, a source line driver circuit, or the like) which suppliesa signal to a source signal line.

Note that categories of a display device, a semiconductor device, alighting device, a cooling device, a light-emitting device, a reflectivedevice, a driving device, and the like overlap with each other in somecases. For example, a display device includes a semiconductor device anda light-emitting device in some cases. Alternatively, a semiconductordevice includes a display device and a driving device in some cases.

When it is explicitly described that B is formed on or over A, it doesnot necessarily mean that B is formed in direct contact with A. Thedescription includes the case where A and B are not in direct contactwith each other, that is, the case where another object is interposedbetween A and B. Here, each of A and B corresponds to an object (e.g., adevice, an element, a circuit, a wiring, an electrode, a terminal, aconductive film, or a layer).

Accordingly, for example, when it is explicitly described that a layer Bis formed on (or over) a layer A, it includes both the case where thelayer B is formed in direct contact with the layer A, and the case whereanother layer (e.g., a layer C or a layer D) is formed in direct contactwith the layer A and the layer B is formed in direct contact with thelayer C or D. Note that another layer (e.g., the layer C or the layer D)may be a single layer or a plurality of layers.

Similarly, when it is explicitly described that B is formed above A, itdoes not necessarily mean that B is in direct contact with A, andanother object may be interposed between A and B. For example, when itis explicitly described that a layer B is formed above a layer A, itincludes both the case where the layer B is formed in direct contactwith the layer A, and the case where another layer (e.g., a layer C or alayer D) is formed in direct contact with the layer A and the layer B isformed in direct contact with the layer C or D. Note that another layer(e.g., the layer C or the layer D) may be a single layer or a pluralityof layers.

When it is explicitly described that B is formed in direct contact withA, it includes the case where B is formed in direct contact with A anddoes not include the case where another object is interposed between Aand B.

Note that the same can be said when it is explicitly described that B isformed below or under A.

Note that explicit singular forms are preferably singular forms.However, without being limited thereto, such singular forms can includeplural forms. Similarly, explicit plural forms are preferably pluralforms. However, without being limited thereto, such plural forms caninclude singular forms.

A frame rate of input image data can be converted into a variety ofdisplay frame rates, so that optimal display frame rate and drivingmethod can be selected depending on circumstances. Accordingly, adisplay device with improved image quality and a driving method thereofcan be obtained.

In addition, when a display frame rate is made larger than a frame rateof input image data or pseudo impulse driving is performed, for example,a display device without the problem of motion blur and a driving methodthereof can be obtained. When a cycle of change in brightness isshortened or the amount of change is reduced, for example, a displaydevice without flickers and a driving method thereof can be obtained.Further, particularly in a liquid crystal display device, when anoptimal liquid crystal mode is selected, for example, a display devicewith wider viewing angles and a driving method thereof can be obtained.Moreover, particularly in a liquid crystal display device, when voltagewhose level is larger than original voltage is temporarily applied tothe device, for example, a display device with higher response speed anda driving method thereof can be obtained. By a variety of methods, forexample, when brightness is suppressed or driving frequency is reduced,a display device with reduced power consumption in addition to theabove-described effects and a driving method thereof can be obtained.Further, by a variety of methods, for example, when a circuit for whichhigh driving frequency is not necessary is used or a manufacturingprocess is simplified, a display device with reduced manufacturing costand a driving method thereof can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B each illustrate one mode of a display device of theinvention.

FIGS. 2A and 2B each illustrate one mode of a display device of theinvention.

FIGS. 3A and 3B each illustrate one mode of a display device of theinvention.

FIGS. 4A to 4F each illustrate one mode of a display device of theinvention.

FIGS. 5A to 5F each illustrate one mode of a display device of theinvention.

FIGS. 6A to 6F each illustrate one mode of a display device of theinvention.

FIGS. 7A and 7B each illustrate one mode of a display device of theinvention.

FIGS. 8A to 8D each illustrate one mode of a display device of theinvention.

FIGS. 9A to 9F each illustrate one mode of a display device of theinvention.

FIGS. 10A to 10D each illustrate one mode of a display device of theinvention.

FIGS. 11A and 11B each illustrate one mode of a display device of theinvention.

FIGS. 12A to 12D each illustrate one mode of a display device of theinvention.

FIG. 13 illustrates one mode of a display device of the invention.

FIG. 14 illustrates one mode of a display device of the invention.

FIG. 15 illustrates one mode of a display device of the invention.

FIGS. 16A to 16C each illustrate one mode of a display device of theinvention.

FIGS. 17A to 17E each illustrate one mode of a display device of theinvention.

FIGS. 18A and 18B each illustrate one mode of a display device of theinvention.

FIGS. 19A to 19D each illustrate one mode of a display device of theinvention.

FIGS. 20A to 20E each illustrate one mode of a display device of theinvention.

FIGS. 21A to 21C each illustrate one mode of a display device of theinvention.

FIGS. 22A to 22E each illustrate one mode of a display device of theinvention.

FIG. 23 illustrates one mode of a display device of the invention.

FIGS. 24A to 24G illustrate one mode of a display device of theinvention.

FIGS. 25A to 25C illustrate one mode of a display device of theinvention.

FIGS. 26A to 26E illustrate one mode of a display device of theinvention.

FIGS. 27A and 27B illustrate one mode of a display device of theinvention.

FIGS. 28A to 28G are cross-sectional views of a display device accordingto the invention.

FIG. 29 is a cross-sectional view of a display device according to theinvention.

FIG. 30 is a cross-sectional view of a display device according to theinvention.

FIG. 31 is a cross-sectional view of a display device according to theinvention.

FIG. 32 is a cross-sectional view of a display device according to theinvention.

FIGS. 33A to 33C are cross-sectional views of a display device accordingto the invention.

FIGS. 34A to 34D are cross-sectional views of a display device accordingto the invention.

FIGS. 35A to 35C are cross-sectional views of a display device accordingto the invention.

FIGS. 36A to 36D are cross-sectional views of a display device accordingto the invention.

FIGS. 37A to 37D are cross-sectional views of a display device accordingto the invention.

FIGS. 38A to 38C each illustrate a structure of a peripheral circuit ofa display device according to the invention.

FIGS. 39A and 39B each illustrate a structure of a peripheral circuit ofa display device according to the invention.

FIG. 40 illustrates a structure of a peripheral circuit of a displaydevice according to the invention.

FIGS. 41A and 41B each illustrate a structure of a peripheral circuit ofa display device according to the invention.

FIG. 42 illustrates a circuit structure of a display device according tothe invention.

FIG. 43 is a timing chart of a display device according to theinvention.

FIG. 44 is a timing chart of a display device according to theinvention.

FIG. 45 is a cross-sectional view of a display device according to theinvention.

FIGS. 46A to 46D each illustrate a peripheral component of a displaydevice according to the invention.

FIG. 47 is a cross-sectional view of a display device according to theinvention.

FIGS. 48A to 48C each are a block diagram of a display device accordingto the invention.

FIGS. 49A and 49B each are a cross-sectional view of a display deviceaccording to the invention.

FIGS. 50A and 50B each illustrate a circuit structure of a displaydevice according to the invention.

FIG. 51 illustrates a circuit structure of a display device according tothe invention.

FIG. 52 illustrates a circuit structure of a display device according tothe invention.

FIGS. 53A to 53E each illustrate a driving method of a display deviceaccording to the invention.

FIGS. 54A and 54B each illustrate a driving method of a display deviceaccording to the invention.

FIGS. 55A to 55C each illustrate a driving method of a display deviceaccording to the invention.

FIGS. 56A to 56C each illustrate a driving method of a display deviceaccording to the invention.

FIGS. 57A to 57C each illustrate a driving method of a display deviceaccording to the invention.

FIGS. 58A and 58B are cross-sectional views of a display deviceaccording to the invention.

FIGS. 59A to 59D are cross-sectional views of display devices accordingto the invention.

FIGS. 60A to 60D are cross-sectional views of display devices accordingto the invention.

FIGS. 61A to 61D are cross-sectional views of display devices accordingto the invention.

FIG. 62 is a top plan view of a display device according to theinvention.

FIGS. 63A to 63D each are a top plan view of a display device accordingto the invention.

FIGS. 64A to 64D each are a top plan view of a display device accordingto the invention.

FIG. 65 is a cross-sectional view of a display device according to theinvention.

FIGS. 66A and 66B each are a cross-sectional view of a display deviceaccording to the invention.

FIGS. 67A and 67B each are a cross-sectional view of a display deviceaccording to the invention.

FIG. 68 is a top plan view of a display device according to theinvention.

FIGS. 69A and 69B each are a top plan view of a display device accordingto the invention.

FIGS. 70A and 70B each are a top plan view of a display device accordingto the invention.

FIGS. 71A to 71E are cross-sectional views of a display device accordingto the invention.

FIGS. 72A and 72B are top plan views of a display device according tothe invention, and FIG. 72C is a cross-sectional view thereof.

FIGS. 73A to 73D are cross-sectional views of a display device accordingto the invention.

FIGS. 74A and 74C are cross-sectional views of a display deviceaccording to the invention, and FIG. 74B is a top plan view thereof.

FIGS. 75A and 75B each are a timing chart of a display device accordingto the invention.

FIGS. 76A and 76B each are a timing chart of a display device accordingto the invention.

FIG. 77 illustrates a circuit structure of a display device according tothe invention.

FIG. 78 illustrates a circuit structure of a display device according tothe invention.

FIG. 79 illustrates a circuit structure of a display device according tothe invention.

FIG. 80 illustrates a circuit structure of a display device according tothe invention.

FIG. 81 illustrates a circuit structure of a display device according tothe invention.

FIG. 82A is a top plan view of a display device according to theinvention, and

FIG. 82B is a cross-sectional view thereof.

FIG. 83A is a top plan view of a display device according to theinvention, and

FIG. 83B is a cross-sectional view thereof.

FIG. 84A is a top plan view of a display device according to theinvention, and

FIG. 84B is a cross-sectional view thereof.

FIGS. 85A to 85E each illustrate a light-emitting element of a displaydevice according to the invention.

FIG. 86 illustrates a manufacturing device of a display device accordingto the invention.

FIG. 87 illustrates a manufacturing device of a display device accordingto the invention.

FIGS. 88A to 88C each illustrate a light-emitting element of a displaydevice according to the invention.

FIGS. 89A to 89C each illustrate a light-emitting element of a displaydevice according to the invention.

FIGS. 90A and 90B illustrate a structure of a display device accordingto the invention.

FIG. 91 illustrates a structure of a display device according to theinvention.

FIG. 92 illustrates a structure of a display device according to theinvention.

FIG. 93 illustrates a structure of a display device according to theinvention.

FIGS. 94A to 94C each illustrate a structure of a display deviceaccording to the invention.

FIG. 95 illustrates an electronic device in which a display deviceaccording to the invention is used.

FIG. 96 illustrates an electronic device in which a display deviceaccording to the invention is used.

FIGS. 97A and 97B each illustrate an electronic device in which adisplay device according to the invention is used.

FIGS. 98A and 98B each illustrate an electronic device in which adisplay device according to the invention is used.

FIG. 99 illustrates an electronic device in which a display deviceaccording to the invention is used.

FIG. 100 illustrates an electronic device in which a display deviceaccording to the invention is used.

FIGS. 101A to 101C each illustrate an electronic device in which adisplay device according to the invention is used.

FIG. 102 illustrates an electronic device in which a display deviceaccording to the invention is used.

FIG. 103 illustrates an electronic device in which a display deviceaccording to the invention is used.

FIG. 104 illustrates an electronic device in which a display deviceaccording to the invention is used.

FIG. 105 illustrates an electronic device in which a display deviceaccording to the invention is used.

FIGS. 106A and 106B each illustrate an electronic device in which adisplay device according to the invention is used.

FIGS. 107A and 107B illustrate an electronic device in which a displaydevice according to the invention is used.

FIGS. 108A to 108C each illustrate an electronic device in which adisplay device according to the invention is used.

FIGS. 109A and 109B each illustrate an electronic device in which adisplay device according to the invention is used.

FIG. 110 illustrates an electronic device in which a display deviceaccording to the invention is used.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiment modes of the present invention will be describedwith reference to drawings. However, the present invention can beimplemented in various modes, and it is easily understood by thoseskilled in the art that modes and details can be variously changedwithout departing from the scope and the spirit of the presentinvention. Therefore, the present invention is not construed as beinglimited to description of the embodiment modes.

Embodiment Mode 1

In this embodiment mode, an example of a method for improving quality ofimages displayed on a display device is described.

In this embodiment mode, a frame rate (the number of frames per second,unit: Hz, also referred to as input frame rate) of image data to beinput does not necessarily correspond to a frame rate of display (alsoreferred to as a display frame rate). When an input frame rate and adisplay frame rate are different from each other, the input frame ratecan be converted by a circuit which converts a frame rate of image data(a frame rate conversion circuit) so that the input frame ratecorresponds to the display frame rate. In such a manner, even when theinput frame rate and the display frame rate are different from eachother, display can be performed at a variety of display frame rates.

When the input frame rate is higher than the display frame rate, part ofthe image data to be input is discarded and the input frame rate isconverted so that display is performed at a variety of display framerates. In this case, the display frame rate can be reduced; thus,operating frequency of a driver circuit used for display can be reduced,and power consumption can be reduced. On the other hand, when the inputframe rate is lower than the display frame rate, display can beperformed at a variety of converted display frame rates by a method suchas a method in which all or part of the image data to be input isdisplayed more than once, a method in which another image is generatedfrom the image data to be input, or a method in which an image having norelation to the image data to be input is generated. In this case,quality of moving images can be improved by the display frame rate beingincreased.

In this embodiment mode, a frame rate conversion method in the casewhere the input frame rate is lower than the display frame rate isdescribed in detail. Note that a frame rate conversion method in thecase where the input frame rate is higher than the display frame ratecan be realized by performance of the frame rate conversion method inthe case where the input frame rate is lower than the display frame ratein reverse order.

In this embodiment mode, an image displayed at the same frame rate asthe input frame rate is referred to as a basic image. An image which isdisplayed at a frame rate different from that of the basic image anddisplayed to ensure that the input frame rate and the display frame rateare consistent to each other is referred to as an interpolation image.As the basic image, the same image as that of the image data to be inputcan be used. As the interpolation image, the same image as the basicimage can be used. Further, an image different from the basic image canbe generated, and the generated image can be used as the interpolationimage.

In order to generate the interpolation image, the following methods canbe used, for example: a method in which temporal change (movement ofimages) of the image data to be input is detected and an image in anintermediate state between the images is employed as the interpolationimage, a method in which an image obtained by multiplication ofluminance of the basic image by a coefficient is employed as theinterpolation image, and a method in which a plurality of differentimages are generated from the image data to be input and the pluralityof images are continuously displayed (one of the plurality of images isemployed as the basic image and the other images are employed asinterpolation images) so as to allow a viewer to perceive an imagecorresponding to the image data to be input. Examples of the method inwhich a plurality of different images are generated from the image datato be input include a method in which a gamma value of the image data tobe input is converted and a method in which a gray scale value includedin the image data to be input is divided up.

Note that an image in an intermediate state (an intermediate image)refers to an image obtained by detection of temporal change (movement ofimages) of the image data to be input and interpolation of the detectedmovement. Obtaining an intermediate image by such a method is referredto as motion compensation.

Next, a specific example of a frame rate conversion method is described.With this method, frame rate conversion multiplied by a given rationalnumber (n/m) can be realized. Here, each of n and m is an integer equalto or more than 1. A frame rate conversion method in this embodimentmode can be treated as being divided into a first step and a secondstep. The first step is a step in which a frame rate is converted bybeing multiplied by the given rational number (n/m). As theinterpolation image, the basic image or the intermediate image obtainedby motion compensation may be used. The second step is a step in which aplurality of different images (sub-images) are generated from the imagedata to be input or from images each of which frame rate is converted inthe first step and the plurality of sub-images are continuouslydisplayed. By use of a method of the second step, human eyes can be madeto perceive display such that the display appears to be an originalimage, despite the fact that a plurality of different images aredisplayed.

Note that in the frame rate conversion method in this embodiment mode,both the first and second steps can be used, the second step only can beused with the first step omitted, or the first step only can be usedwith the second step omitted.

First, as the first step, frame rate conversion multiplied by the givenrational number (n/m) is described with reference to FIG. 13. In FIG.13, the horizontal axis represents time, and the vertical axisrepresents cases for various combinations of n and m. Each pattern inFIG. 13 is a schematic diagram of an image to be displayed, and ahorizontal position of the pattern represents timing of display. A dotin the pattern schematically represents movement of an image. Note thateach of these images is an example for explanation, and an image to bedisplayed is not limited to one of these images. This method can beapplied to a variety of images.

The period T_(in) represents a cycle of input image data. The cycle ofinput image data corresponds to an input frame rate. For example, whenthe input frame rate is 60 Hz, the cycle of input image data is 1/60seconds. Similarly, when the input frame rate is 50 Hz, the cycle ofinput image data is 1150 seconds. Accordingly, the cycle (unit: second)of input image data is an inverse number of the input frame rate (unit:Hz). Note that a variety of input frame rates such as 24 Hz, 50 Hz, 60Hz, 70 Hz, 48 Hz, 100 Hz, 120 Hz, and 140 Hz can be used. 24 Hz is aframe rate for movies on film, for example. 50 Hz is a frame rate for avideo signal of the PAL standard, for example. 60 Hz is a frame rate fora video signal of the NTSC standard, for example. 70 Hz is a frame rateof a display input signal of a personal computer, for example. 48 Hz,100 Hz, 120 Hz, and 140 Hz are twice as high as 24 Hz, 50 Hz, 60 Hz, and70 Hz, respectively. Note that the frame rate can not only be doubledbut also multiplied by a variety of numbers. As described above, withthe method shown in this embodiment mode, a frame rate can be convertedwith respect to an input signal of various standards. Procedures offrame rate conversion multiplied by the given rational number (n/m)times in the first step are as follows. As a procedure 1, display timingof a k-th interpolation image (k is an integer equal to or more than 1,where the initial value is 1) with respect to a first basic image isdecided. The display timing of the k-th interpolation image is at thetiming of passage of a period obtained by multiplication of the cycle ofinput image data by k(m/n) after the first basic image is displayed. Asa procedure 2, whether the coefficient k(m/n) used for deciding thedisplay timing of the k-th interpolation image is an integer or not isdetermined. When the coefficient k is an integer, a (k(m/n)+1)th basicimage is displayed at the display timing of the k-th interpolationimage, and the first step is finished. When the coefficient k is not aninteger, the operation proceeds to a procedure 3. As the procedure 3, animage used as the k-th interpolation image is decided. Specifically, thecoefficient k(m/n) used for deciding the display timing of the k-thinterpolation image is converted into the form x+y/n. Each of x and y isan integer, and y is smaller than n. When an intermediate image obtainedby motion compensation is employed as the k-th interpolation image, anintermediate image which is an image corresponding to movement obtainedby multiplication of the amount of movement from an (x+1)th basic imageto an (x+2)th basic image by (y/n) is employed as the k-th interpolationimage. When the k-th interpolation image is the same image as the basicimage, the (x+1)th basic image can be used. Note that a method forobtaining an intermediate image as an image corresponding to movementobtained by multiplication of the amount of movement of the image by(y/n) will be described in detail later. As a procedure 4, a nextinterpolation image is set to be the objective interpolation image.Specifically, the value of k is increased by one, and the operationreturns to the procedure 1.

Next, the procedures in the first step are described in detail usingspecific values of n and m.

Note that a mechanism for performing the procedures in the first stepmay be mounted on a device or decided in the design phase of the devicein advance. When the mechanism for performing the procedures in thefirst step is mounted on the device, a driving method can be switched sothat optimal operations depending on circumstances can be performed.Note that the circumstances here include contents of image data,environment inside and outside the device (e.g., temperature, humidity,barometric pressure, light, sound, electric field, the amount ofradiation, altitude, acceleration, or movement speed), user settings,software version, and the like. On the other hand, when the mechanismfor performing the procedures in the first step is decided in the designphase of the device in advance, driver circuits optimal for respectivedriving methods can be used. Moreover, since the mechanism is decided,manufacturing cost can be reduced due to efficiency of mass production.

When n=1 and m=1, that is, when a conversion ratio (n/m) is 1 (where n=1and m=1 in FIG. 13), an operation in the first step is as follows. Whenk=1, in the procedure 1, display timing of a first interpolation imagewith respect to the first basic image is decided. The display timing ofthe first interpolation image is at the timing of passage of a periodobtained by multiplication of the length of the cycle of input imagedata by k(m/n), that is, 1 after the first basic image is displayed.

Next, in the procedure 2, whether the coefficient k(m/n) used fordeciding the display timing of the first interpolation image is aninteger or not is determined. Here, the coefficient k(m/n) is 1, whichis an integer. Consequently, the (k(m/n)+1)th basic image, that is, asecond basic image is displayed at the display timing of the firstinterpolation image, and the first step is finished.

In other words, when the conversion ratio is 1, the k-th image is abasic image, the (k+1)th image is a basic image, and an image displaycycle is equal to the cycle of input image data.

Specifically, in a driving method of a display device in which, when theconversion ratio is 1 (n/m=1), i-th image data (i is a positive integer)and (i+1)th image data are sequentially input as input image data in acertain cycle and the k-th image (k is a positive integer) and the(k+1)th image are sequentially displayed at an interval equal to thecycle of the input image data, the k-th image is displayed in accordancewith the i-th image data, and the (k+1)th image is displayed inaccordance with the (i+1)th image data.

Since the frame rate conversion circuit can be omitted when theconversion ratio is 1, manufacturing cost can be reduced. Further, whenthe conversion ratio is 1, quality of moving images can be improvedcompared with the case where the conversion ratio is less than 1.Moreover, when the conversion ratio is 1, power consumption andmanufacturing cost can be reduced compared with the case where theconversion ratio is more than 1.

When n=2 and m=1, that is, when the conversion ratio (n/m) is 2 (wheren=2 and m=1 in FIG. 13), an operation in the first step is as follows.When k=1, in the procedure 1, display timing of the first interpolationimage with respect to the first basic image is decided. The displaytiming of the first interpolation image is at the timing of passage of aperiod obtained by multiplication of the length of the cycle of inputimage data by k(m/n), that is, 1/2 after the first basic image isdisplayed.

Next, in the procedure 2, whether the coefficient k(m/n) used fordeciding the display timing of the first interpolation image is aninteger or not is determined. Here, the coefficient k(m/n) is 1/2, whichis not an integer. Consequently, the operation proceeds to the procedure3.

In the procedure 3, an image used as the first interpolation image isdecided. In order to decide the image, the coefficient 1/2 is convertedinto the form x+y/n. In the case of the coefficient 1/2, x=0 and y=1.When an intermediate image obtained by motion compensation is employedas the first interpolation image, an intermediate image corresponding tomovement obtained by multiplication of the amount of movement from the(x+1)th basic image, that is, the first basic image to the (x+2)th basicimage, that is, the second basic image by (y/n), that is, 1/2 isemployed as the first interpolation image. When the first interpolationimage is the same image as the basic image, the (x+1)th basic image,that is, the first basic image can be used.

According to the procedures performed up to this point, the displaytiming of the first interpolation image and the image displayed as thefirst interpolation image can be decided. Next, in the procedure 4, theobjective interpolation image is shifted from the first interpolationimage to a second interpolation image. That is, k is changed from 1 to2, and the operation returns to the procedure 1.

When k=2, in the procedure 1, display timing of the second interpolationimage with respect to the first basic image is decided. The displaytiming of the second interpolation image is at the timing of passage ofa period obtained by multiplication of the length of the cycle of inputimage data by k(m/n), that is, 1 after the first basic image isdisplayed.

Next, in the procedure 2, whether the coefficient k(m/n) used fordeciding the display timing of the second interpolation image is aninteger or not is determined. Here, the coefficient k(m/n) is 1, whichis an integer. Consequently, the (k(m/n)+1)th basic image, that is, thesecond basic image is displayed at the display timing of the secondinterpolation image, and the first step is finished.

In other words, when the conversion ratio is 2 (n/m=2), the k-th imageis a basic image, the (k+1)th image is an interpolation image, a (k+2)thimage is a basic image, and an image display cycle is half the cycle ofinput image data.

Specifically, in a driving method of a display device in which, when theconversion ratio is 2 (n/m=2), the i-th image data (i is a positiveinteger) and the (i+1)th image data are sequentially input as inputimage data in a certain cycle and the k-th image (k is a positiveinteger), the (k+1)th image, and the (k+2)th image are sequentiallydisplayed at an interval which is half the cycle of the input imagedata, the k-th image is displayed in accordance with the i-th imagedata, the (k+1)th image is displayed in accordance with image datacorresponding to movement obtained by multiplication of the amount ofmovement from the i-th image data to the (i+1)th image data by 1/2, andthe (k+2)th image is displayed in accordance with the (i+1)th imagedata.

Even specifically, in a driving method of a display device in which,when the conversion ratio is 2 (n/m=2), the i-th image data (i is apositive integer) and the (i+1)th image data are sequentially input asinput image data in a certain cycle and the k-th image (k is a positiveinteger), the (k+1)th image, and the (k+2)th image are sequentiallydisplayed at an interval which is half the cycle of the input imagedata, the k-th image is displayed in accordance with the i-th imagedata, the (k+1)th image is displayed in accordance with the i-th imagedata, and the (k+2)th image is displayed in accordance with the (i+1)thimage data.

Specifically, when the conversion ratio is 2, driving is also referredto as double-frame rate driving. For example, when the input frame rateis 60 Hz, the display frame rate is 120 Hz (120 Hz driving).Accordingly, two images are continuously displayed with respect to oneinput image. At this time, when an interpolation image is anintermediate image obtained by motion compensation, the movement ofmoving images can be made to be smooth; thus, quality of the movingimage can be significantly improved. Further, quality of moving imagescan be significantly improved particularly when the display device is anactive matrix liquid crystal display device. This is related to aproblem of lack of writing voltage due to change in the electrostaticcapacity of a liquid crystal element by applied voltage, so-calleddynamic capacitance. That is, when the display frame rate is made higherthan the input frame rate, the frequency of a writing operation of imagedata can be increased; thus, defects such as an afterimage and aphenomenon of a moving image in which traces are seen due to lack ofwriting voltage because of dynamic capacitance can be reduced. Moreover,a combination of 120 Hz driving and alternating-current driving of aliquid crystal display device is effective. That is, when drivingfrequency of the liquid crystal display device is 120 Hz and frequencyof alternating-current driving is an integer multiple of 120 Hz or aunit fraction of 120 Hz (e.g., 30 Hz, 60 Hz, 120 Hz, or 240 Hz),flickers which appear in alternating-current driving can be reduced to alevel that cannot be perceived by human eyes.

When n=3 and m=1, that is, when the conversion ratio (n/m) is 3 (wheren=3 and m=1 in FIG. 13), an operation in the first step is as follows.First, when k=1, in the procedure 1, display timing of the firstinterpolation image with respect to the first basic image is decided.The display timing of the first interpolation image is at the timing ofpassage of a period obtained by multiplication of the length of thecycle of input image data by k(m/n), that is, 1/3 after the first basicimage is displayed.

Next, in the procedure 2, whether the coefficient k(mm) used fordeciding the display timing of the first interpolation image is aninteger or not is determined. Here, the coefficient k(m/n) is 1/3, whichis not an integer. Consequently, the operation proceeds to the procedure3.

In the procedure 3, an image used as the first interpolation image isdecided. In order to decide the image, the coefficient 1/3 is convertedinto the form x+y/n. In the case of the coefficient 1/3, x=0 and y=1.When an intermediate image obtained by motion compensation is employedas the first interpolation image, an intermediate image corresponding tomovement obtained by multiplication of the amount of movement from the(x+1)th basic image, that is, the first basic image to the (x+2)th basicimage, that is, the second basic image by (y/n), that is, 1/3 isemployed as the first interpolation image. When the first interpolationimage is the same image as the basic image, the (x+1)th basic image,that is, the first basic image can be used.

According to the procedures performed up to this point, the displaytiming of the first interpolation image and the image displayed as thefirst interpolation image can be decided. Next, in the procedure 4, theobjective interpolation image is shifted from the first interpolationimage to the second interpolation image. That is, k is changed from 1 to2, and the operation returns to the procedure 1.

When k=2, in the procedure 1, display timing of the second interpolationimage with respect to the first basic image is decided. The displaytiming of the second interpolation image is at the timing of passage ofa period obtained by multiplication of the length of the cycle of inputimage data by k(m/n), that is, 2/3 after the first basic image isdisplayed.

Next, in the procedure 2, whether the coefficient k(m/n) used fordeciding the display timing of the second interpolation image is aninteger or not is determined. Here, the coefficient k(m/n) is 2/3, whichis not an integer. Consequently, the operation proceeds to the procedure3.

In the procedure 3, an image used as the second interpolation image isdecided. In order to decide the image, the coefficient 2/3 is convertedinto the form x+y/n. In the case of the coefficient 2/3, x=0 and y=2.When an intermediate image obtained by motion compensation is employedas the second interpolation image, an intermediate image correspondingto movement obtained by multiplication of the amount of movement fromthe (x+1)th basic image, that is, the first basic image to the (x+2)thbasic image, that is, the second basic image by (y/n), that is, 2/3 isemployed as the second interpolation image. When the secondinterpolation image is the same image as the basic image, the (x+1)thbasic image, that is, the first basic image can be used.

According to the procedures performed up to this point, the displaytiming of the second interpolation image and the image displayed as thesecond interpolation image can be decided. Next, in the procedure 4, theobjective interpolation image is shifted from the second interpolationimage to a third interpolation image. That is, k is changed from 2 to 3,and the operation returns to the procedure 1.

When k=3, in the procedure 1, display timing of the third interpolationimage with respect to the first basic image is decided. The displaytiming of the third interpolation image is at the timing of passage of aperiod obtained by multiplication of the length of the cycle of inputimage data by k(m/n), that is, 1 after the first basic image isdisplayed.

Next, in the procedure 2, whether the coefficient k(m/n) used fordeciding the display timing of the third interpolation image is aninteger or not is determined. Here, the coefficient k(m/n) is 1, whichis an integer. Consequently, the (k(m/n)+1)th basic image, that is, thesecond basic image is displayed at the display timing of the thirdinterpolation image, and the first step is finished.

In other words, when the conversion ratio is 3 (n/m=3), the k-th imageis a basic image, the (k+1)th image is an interpolation image, the(k+2)th image is an interpolation image, a (k+3)th image is a basicimage, and an image display cycle is 1/3 times the cycle of input imagedata.

Specifically, in a driving method of a display device in which, when theconversion ratio is 3 (n/m=3), the i-th image data (i is a positiveinteger) and the (i+1)th image data are sequentially input as inputimage data in a certain cycle and the k-th image (k is a positiveinteger), the (k+1)th image, the (k+2)th image, and the (k+3)th imageare sequentially displayed at an interval which is 1/3 times the cycleof the input image data, the k-th image is displayed in accordance withthe i-th image data, the (k+1)th image is displayed in accordance withimage data corresponding to movement obtained by multiplication of theamount of movement from the i-th image data to the (i+1)th image data by1/3, the (k+2)th image is displayed in accordance with image datacorresponding to movement obtained by multiplication of the amount ofmovement from the i-th image data to the (i+1)th image data by 2/3, andthe (k+3)th image is displayed in accordance with the (i+1)th imagedata.

Even specifically, in a driving method of a display device in which,when the conversion ratio is 3 (n/m=3), the i-th image data (i is apositive integer) and the (i+1)th image data are sequentially input asinput image data in a certain cycle and the k-th image (k is a positiveinteger), the (k+1)th image, the (k+2)th image, and the (k+3)th imageare sequentially displayed at an interval which is 1/3 times the cycleof the input image data, the k-th image is displayed in accordance withthe i-th image data, the (k+1)th image is displayed in accordance withthe i-th image data, the (k+2)th image is displayed in accordance withthe i-th image data, and the (k+3)th image is displayed in accordancewith the (i+1)th image data.

When the conversion ratio is 3, quality of moving images can be improvedcompared with the case where the conversion ratio is less than 3.Moreover, when the conversion ratio is 3, power consumption andmanufacturing cost can be reduced compared with the case where theconversion ratio is more than 3.

Specifically, when the conversion ratio is 3, driving is also referredto as triple-frame rate driving. For example, when the input frame rateis 60 Hz, the display frame rate is 180 Hz (180 Hz driving).Accordingly, three images are continuously displayed with respect to oneinput image. At this time, when an interpolation image is anintermediate image obtained by motion compensation, the movement ofmoving images can be made to be smooth; thus, quality of the movingimage can be significantly improved. Further, when the display device isan active matrix liquid crystal display device, a problem of lack ofwriting voltage due to dynamic capacitance can be avoided; thus, qualityof moving images can be significantly improved, in particular withrespect to defects such as an afterimage and a phenomenon of a movingimage in which traces are seen. Moreover, a combination of 180 Hzdriving and alternating-current driving of a liquid crystal displaydevice is effective. That is, when driving frequency of the liquidcrystal display device is 180 Hz and frequency of alternating-currentdriving is an integer multiple of 180 Hz or a unit fraction of 180 Hz(e.g., 45 Hz, 90 Hz, 180 Hz, or 360 Hz), flickers which appear inalternating-current driving can be reduced to a level that cannot beperceived by human eyes.

When n=3 and m=2, that is, when the conversion ratio (n/m) is 3/2 (wheren=3 and m=2 in FIG. 13), an operation in the first step is as follows.When k=1, in the procedure 1, the display timing of the firstinterpolation image with respect to the first basic image is decided.The display timing of the first interpolation image is at the timing ofpassage of a period obtained by multiplication of the length of thecycle of input image data by k(m/n), that is, 2/3 after the first basicimage is displayed.

Next, in the procedure 2, whether the coefficient k(m/n) used fordeciding the display timing of the first interpolation image is aninteger or not is determined. Here, the coefficient k(m/n) is 2/3, whichis not an integer. Consequently, the operation proceeds to the procedure3.

In the procedure 3, an image used as the first interpolation image isdecided. In order to decide the image, the coefficient 2/3 is convertedinto the form x+y/n. In the case of the coefficient 2/3, x=0 and y=2.When an intermediate image obtained by motion compensation is employedas the first interpolation image, an intermediate image corresponding tomovement obtained by multiplication of the amount of movement from the(x+1)th basic image, that is, the first basic image to the (x+2)th basicimage, that is, the second basic image by (y/n), that is, 2/3 isemployed as the first interpolation image. When the first interpolationimage is the same image as the basic image, the (x+1)th basic image,that is, the first basic image can be used.

According to the procedures performed up to this point, the displaytiming of the first interpolation image and the image displayed as thefirst interpolation image can be decided. Next, in the procedure 4, theobjective interpolation image is shifted from the first interpolationimage to the second interpolation image. That is, k is changed from 1 to2, and the operation returns to the procedure 1.

When k=2, in the procedure 1, the display timing of the secondinterpolation image with respect to the first basic image is decided.The display timing of the second interpolation image is at the timing ofpassage of a period obtained by multiplication of the length of thecycle of input image data by k(m/n), that is, 4/3 after the first basicimage is displayed.

Next, in the procedure 2, whether the coefficient k(m/n) used fordeciding the display timing of the second interpolation image is aninteger or not is determined. Here, the coefficient k(m/n) is 4/3, whichis not an integer. Consequently, the operation proceeds to the procedure3.

In the procedure 3, an image used as the second interpolation image isdecided. In order to decide the image, the coefficient 4/3 is convertedinto the form x+y/n. In the case of the coefficient 4/3, x=1 and y=1.When an intermediate image obtained by motion compensation is employedas the second interpolation image, an intermediate image correspondingto movement obtained by multiplication of the amount of movement fromthe (x+1)th basic image, that is, the second basic image to the (x+2)thbasic image, that is, a third basic image by (y/n), that is, 1/3 isemployed as the second interpolation image. When the secondinterpolation image is the same image as the basic image, the (x+1)thbasic image, that is, the second basic image can be used.

According to the procedures performed up to this point, the displaytiming of the second interpolation image and the image displayed as thesecond interpolation image can be decided. Next, in the procedure 4, theobjective interpolation image is shifted from the second interpolationimage to the third interpolation image. That is, k is changed from 2 to3, and the operation returns to the procedure 1.

When k=3, in the procedure 1, the display timing of the thirdinterpolation image with respect to the first basic image is decided.The display timing of the third interpolation image is at the timing ofpassage of a period obtained by multiplication of the length of thecycle of input image data by k(m/n), that is, 2 after the first basicimage is displayed.

Next, in the procedure 2, whether the coefficient k(mm) used fordeciding the display timing of the third interpolation image is aninteger or not is determined. Here, the coefficient k(m/n) is 2, whichis an integer. Consequently, the (k(m/n)+1)th basic image, that is, thethird basic image is displayed at the display timing of the thirdinterpolation image, and the first step is finished.

In other words, when the conversion ratio is 3/2 (n/m=3/2), the k-thimage is a basic image, the (k+1)th image is an interpolation image, the(k+2)th image is an interpolation image, the (k+3)th image is a basicimage, and an image display cycle is 2/3 times the cycle of input imagedata.

Specifically, in a driving method of a display device in which, when theconversion ratio is 3/2 (n/m=3/2), the i-th image data (i is a positiveinteger), the (i+1)th image data, and (i+2)th image data aresequentially input as input image data in a certain cycle and the k-thimage (k is a positive integer), the (k+1)th image, the (k+2)th image,and the (k+3)th image are sequentially displayed at an interval which is2/3 times the cycle of the input image data, the k-th image is displayedin accordance with the i-th image data, the (k+1)th image is displayedin accordance with image data corresponding to movement obtained bymultiplication of the amount of movement from the i-th image data to the(i+1)th image data by 2/3, the (k+2)th image is displayed in accordancewith image data corresponding to movement obtained by multiplication ofthe amount of movement from the (i+1)th image data to the (i+2)th imagedata by 1/3, and the (k+3)th image is displayed in accordance with the(i+2)th image data.

Even specifically, in a driving method of a display device in which,when the conversion ratio is 3/2 (n/m=3/2), the i-th image data (i is apositive integer), the (i+1)th image data, and the (i+2)th image dataare sequentially input as input image data in a certain cycle and thek-th image (k is a positive integer), the (k+1)th image, the (k+2)thimage, and the (k+3)th image are sequentially displayed at an intervalwhich is 2/3 times the cycle of the input image data, the k-th image isdisplayed in accordance with the i-th image data, the (k+1)th image isdisplayed in accordance with the i-th image data, the (k+2)th image isdisplayed in accordance with the (i+1)th image data, and the (k+3)thimage is displayed in accordance with the (i+2)th image data.

When the conversion ratio is 3/2, quality of moving images can beimproved compared with the case where the conversion ratio is less than3/2. Moreover, when the conversion ratio is 3/2, power consumption andmanufacturing cost can be reduced compared with the case where theconversion ratio is more than 3/2.

Specifically, when the conversion ratio is 3/2, driving is also referredto as 3/2-fold frame rate driving or 1.5-fold frame rate driving. Forexample, when the input frame rate is 60 Hz, the display frame rate is90 Hz (90 Hz driving). Accordingly, three images are continuouslydisplayed with respect to two input images. At this time, when aninterpolation image is an intermediate image obtained by motioncompensation, the movement of moving images can be made to be smooth;thus, quality of the moving image can be significantly improved.Moreover, operating frequency of a circuit used for obtaining anintermediate image by motion compensation can be reduced, in particular,compared with a driving method with high driving frequency, such as 120Hz driving (double-frame rate driving) or 180 Hz driving (triple-framerate driving); thus, an inexpensive circuit can be used, andmanufacturing cost and power consumption can be reduced. Further, whenthe display device is an active matrix liquid crystal display device, aproblem of lack of writing voltage due to dynamic capacitance can beavoided; thus, quality of moving images can be significantly improved,in particular with respect to defects such as an afterimage and aphenomenon of a moving image in which traces are seen. Moreover, acombination of 90 Hz driving and alternating-current driving of a liquidcrystal display device is effective. That is, when driving frequency ofthe liquid crystal display device is 90 Hz and frequency ofalternating-current driving is an integer multiple of 90 Hz or a unitfraction of 90 Hz (e.g., 30 Hz, 45 Hz, 90 Hz, or 180 Hz), flickers whichappear in alternating-current driving can be reduced to a level thatcannot be perceived by human eyes.

Detailed description of procedures for positive integers n and m otherthan those described above is omitted. A conversion ratio can be set asa given rational number (n/m) in accordance with the procedures of framerate conversion in the first step. Note that among combinations of thepositive integers n and m, a combination in which a conversion ratio(n/m) can be reduced to its lowest term can be treated the same as aconversion ratio that is already reduced to its lowest term.

For example, when n=4 and m=1, that is, when the conversion ratio (n/m)is 4 (where n=4 and m=1 in FIG. 13), the k-th image is a basic image,the (k+1)th image is an interpolation image, the (k+2)th image is aninterpolation image, the (k+3)th image is an interpolation image, a(k+4)th image is a basic image, and an image display cycle is 1/4 timesthe cycle of input image data.

Specifically, in a driving method of a display device in which, when theconversion ratio is 4 (n/m=4), the i-th image data (i is a positiveinteger) and the (i+1)th image data are sequentially input as inputimage data in a certain cycle and the k-th image (k is a positiveinteger), the (k+1)th image, the (k+2)th image, the (k+3)th image, andthe (k+4)th image are sequentially displayed at an interval which is 1/4times the cycle of the input image data, the k-th image is displayed inaccordance with the i-th image data, the (k+1)th image is displayed inaccordance with image data corresponding to movement obtained bymultiplication of the amount of movement from the i-th image data to the(i+1)th image data by 1/4, the (k+2)th image is displayed in accordancewith image data corresponding to movement obtained by multiplication ofthe amount of movement from the i-th image data to the (i+1)th imagedata by 1/2, the (k+3)th image is displayed in accordance with imagedata corresponding to movement obtained by multiplication of the amountof movement from the i-th image data to the (i+1)th image data by 3/4,and the (k+4)th image is displayed in accordance with the (i+1)th imagedata.

Even specifically, in a driving method of a display device in which,when the conversion ratio is 4 (n/m=4), the i-th image data (i is apositive integer) and the (i+1)th image data are sequentially input asinput image data in a certain cycle and the k-th image (k is a positiveinteger), the (k+1)th image, the (k+2)th image, the (k+3)th image, andthe (k+4)th image are sequentially displayed at an interval which is 1/4times the cycle of the input image data, the k-th image is displayed inaccordance with the i-th image data, the (k+1)th image is displayed inaccordance with the i-th image data, the (k+2)th image is displayed inaccordance with the i-th image data, the (k+3)th image is displayed inaccordance with the i-th image data, and the (k+4)th image is displayedin accordance with the (i+1)th image data.

When the conversion ratio is 4, quality of moving images can be improvedcompared with the case where the conversion ratio is less than 4.Moreover, when the conversion ratio is 4, power consumption andmanufacturing cost can be reduced compared with the case where theconversion ratio is more than 4.

Specifically, when the conversion ratio is 4, driving is also referredto as quadruple-frame rate driving. For example, when the input framerate is 60 Hz, the display frame rate is 240 Hz (240 Hz driving).Accordingly, four images are continuously displayed with respect to oneinput image. At this time, when an interpolation image is anintermediate image obtained by motion compensation, the movement ofmoving images can be made to be smooth; thus, quality of the movingimage can be significantly improved. Moreover, an interpolation imageobtained by more accurate motion compensation can be used, inparticular, compared with a driving method with low driving frequency,such as 120 Hz driving (double-frame rate driving) or 180 Hz driving(triple-frame rate driving); thus, the movement of moving images can bemade smoother, and quality of the moving image can be significantlyimproved. Further, when the display device is an active matrix liquidcrystal display device, a problem of lack of writing voltage due todynamic capacitance can be avoided; thus, quality of moving images canbe significantly improved, in particular with respect to defects such asan afterimage and a phenomenon of a moving image in which traces areseen. Moreover, a combination of 240 Hz driving and alternating-currentdriving of a liquid crystal display device is effective. That is, whendriving frequency of the liquid crystal display device is 240 Hz andfrequency of alternating-current driving is an integer multiple of 240Hz or a unit fraction of 240 Hz (e.g., 30 Hz, 40 Hz, 60 Hz, or 120 Hz),flickers which appear in alternating-current driving can be reduced to alevel that cannot be perceived by human eyes.

For example, when n=4 and m=3, that is, when the conversion ratio (n/m)is 4/3 (where n=4 and m=3 in FIG. 13), the k-h image is a basic image,the (k+1)th image is an interpolation image, the (k+2)th image is aninterpolation image, the (k+3)th image is an interpolation image, the(k+4)th image is a basic image, and an image display cycle is 3/4 timesthe cycle of input image data.

Specifically, in a driving method of a display device in which, when theconversion ratio is 4/3 (n/m=4/3), the i-th image data (i is a positiveinteger), the (i+1)th image data, the (i+2)th image data, and (i+3)thimage data are sequentially input as input image data in a certain cycleand the k-th image (k is a positive integer), the (k+1)th image, the(k+2)th image, the (k+3)th image, and the (k+4)th image are sequentiallydisplayed at an interval which is 3/4 times the cycle of the input imagedata, the k-th image is displayed in accordance with the i-th imagedata, the (k+1)th image is displayed in accordance with image datacorresponding to movement obtained by multiplication of the amount ofmovement from the i-th image data to the (i+1)th image data by 3/4, the(k+2)th image is displayed in accordance with image data correspondingto movement obtained by multiplication of the amount of movement fromthe (i+1)th image data to the (i+2)th image data by 1/2, the (k+3)thimage is displayed in accordance with image data corresponding tomovement obtained by multiplication of the amount of movement from the(i+2)th image data to the (i+3)th image data by 1/4, and the (k+4)thimage is displayed in accordance with the (i+3)th image data.

Even specifically, in a driving method of a display device in which,when the conversion ratio is 4/3 (n/m=4/3), the i-th image data (i is apositive integer), the (i+1)th image data, the (i+2)th image data, andthe (i+3)th image data are sequentially input as input image data in acertain cycle and the k-th image (k is a positive integer), the (k+1)thimage, the (k+2)th image, the (k+3)th image, and the (k+4)th image aresequentially displayed at an interval which is 3/4 times the cycle ofthe input image data, the k-th image is displayed in accordance with thei-th image data, the (k+1)th image is displayed in accordance with thei-th image data, the (k+2)th image is displayed in accordance with the(i+1)th image data, the (k+3)th image is displayed in accordance withthe (i+2)th image data, and the (k+4)th image is displayed in accordancewith the (i+3)th image data.

When the conversion ratio is 4/3, quality of moving images can beimproved compared with the case where the conversion ratio is less than4/3. Moreover, when the conversion ratio is 4/3, power consumption andmanufacturing cost can be reduced compared with the case where theconversion ratio is more than 4/3.

Specifically, when the conversion ratio is 4/3, driving is also referredto as 4/3-fold frame rate driving or 1.25-fold frame rate driving. Forexample, when the input frame rate is 60 Hz, the display frame rate is80 Hz (80 Hz driving). Accordingly, four images are continuouslydisplayed with respect to three input images. At this time, when aninterpolation image is an intermediate image obtained by motioncompensation, the movement of moving images can be made to be smooth;thus, quality of the moving image can be significantly improved.Moreover, operating frequency of a circuit used for obtaining anintermediate image by motion compensation can be reduced, in particular,compared with a driving method with high driving frequency, such as 120Hz driving (double-frame rate driving) or 180 Hz driving (triple-framerate driving); thus, an inexpensive circuit can be used, andmanufacturing cost and power consumption can be reduced. Further, whenthe display device is an active matrix liquid crystal display device, aproblem of lack of writing voltage due to dynamic capacitance can beavoided; thus, quality of moving images can be significantly improved,in particular with respect to defects such as an afterimage and aphenomenon of a moving image in which traces are seen. Moreover, acombination of 80 Hz driving and alternating-current driving of a liquidcrystal display device is effective. That is, when driving frequency ofthe liquid crystal display device is 80 Hz and frequency ofalternating-current driving is an integer multiple of 80 Hz or a unitfraction of 80 Hz (e.g., 40 Hz, 80 Hz, 160 Hz, or 240 Hz), flickerswhich appear in alternating-current driving can be reduced to a levelthat cannot be perceived by human eyes.

For example, when n=5 and m=1, that is, when the conversion ratio (n/m)is 5 (where n=5 and m=1 in FIG. 13), the k-th image is a basic image,the (k+1)th image is an interpolation image, the (k+2)th image is aninterpolation image, the (k+3)th image is an interpolation image, the(k+4)th image is an interpolation image, and a (k+5)th image is a basicimage, and an image display cycle is 1/5 times the cycle of input imagedata.

Specifically, in a driving method of a display device in which, when theconversion ratio is 5 (n/m=5), the i-th image data (i is a positiveinteger) and the (i+1)th image data are sequentially input as inputimage data in a certain cycle and the k-th image (k is a positiveinteger), the (k+1)th image, the (k+2)th image, the (k+3)th image, the(k+4)th image, and the (k+5)th image are sequentially displayed at aninterval which is 1/5 times the cycle of the input image data, the k-thimage is displayed in accordance with the i-th image data, the (k+1)thimage is displayed in accordance with image data corresponding tomovement obtained by multiplication of the amount of movement from thei-th image data to the (i+1)th image data by 1/5, the (k+2)th image isdisplayed in accordance with image data corresponding to movementobtained by multiplication of the amount of movement from the i-th imagedata to the (i+1)th image data by 2/5, the (k+3)th image is displayed inaccordance with image data corresponding to movement obtained bymultiplication of the amount of movement from the i-th image data to the(i+1)th image data by 3/5, the (k+4)th image is displayed in accordancewith image data corresponding to movement obtained by multiplication ofthe amount of movement from the i-th image data to the (i+1)th imagedata by 4/5, and the (k+5)th image is displayed in accordance with the(i+1)th image data.

Even specifically, in a driving method of a display device in which,when the conversion ratio is 5 (n/m=5), the i-th image data (i is apositive integer) and the (i+1)th image data are sequentially input asinput image data in a certain cycle and the k-th image (k is a positiveinteger), the (k+1)th image, the (k+2)th image, the (k+3)th image, the(k+4)th image, and the (k+5)th image are sequentially displayed at aninterval which is 1/5 times the cycle of the input image data, the k-thimage is displayed in accordance with the i-th image data, the (k+1)thimage is displayed in accordance with the i-th image data, the (k+2)thimage is displayed in accordance with the i-th image data, the (k+3)thimage is displayed in accordance with the i-th image data, the (k+4)thimage is displayed in accordance with the i-th image data, and the(k+5)th image is displayed in accordance with the (i+1)th image data.

When the conversion ratio is 5, quality of moving images can be improvedcompared with the case where the conversion ratio is less than 5.Moreover, when the conversion ratio is 5, power consumption andmanufacturing cost can be reduced compared with the case where theconversion ratio is more than 5.

Specifically, when the conversion ratio is 5, driving is also referredto as 5-fold frame rate driving. For example, when the input frame rateis 60 Hz, the display frame rate is 300 Hz (300 Hz driving).Accordingly, five images are continuously displayed with respect to oneinput image. At this time, when an interpolation image is anintermediate image obtained by motion compensation, the movement ofmoving images can be made to be smooth; thus, quality of the movingimage can be significantly improved. Moreover, an interpolation imageobtained by more accurate motion compensation can be used, inparticular, compared with a driving method with low driving frequency,such as 120 Hz driving (double-frame rate driving) or 180 Hz driving(triple-frame rate driving); thus, the movement of moving images can bemade smoother, and quality of the moving image can be significantlyimproved. Further, when the display device is an active matrix liquidcrystal display device, a problem of lack of writing voltage due todynamic capacitance can be avoided; thus, quality of moving images canbe significantly improved, in particular with respect to defects such asan afterimage and a phenomenon of a moving image in which traces areseen. Moreover, a combination of 300 Hz driving and alternating-currentdriving of a liquid crystal display device is effective. That is, whendriving frequency of the liquid crystal display device is 300 Hz andfrequency of alternating-current driving is an integer multiple of 300Hz or a unit fraction of 300 Hz (e.g., 30 Hz, 50 Hz, 60 Hz, or 100 Hz),flickers which appear in alternating-current driving can be reduced to alevel that cannot be perceived by human eyes.

For example, when n=5 and m=2, that is, when the conversion ratio (n/m)is 5/2 (where n=5 and m=2 in FIG. 13), the k-th image is a basic image,the (k+1)th image is an interpolation image, the (k+2)th image is aninterpolation image, the (k+3)th image is an interpolation image, the(k+4)th image is an interpolation image, and the (k+5)th image is abasic image, and an image display cycle is 2/5 times the cycle of inputimage data.

Specifically, in a driving method of a display device in which, when theconversion ratio is 5/2 (n/m=5/2), the i-th image data (i is a positiveinteger), the (i+1)th image data, and the (i+2)th image data aresequentially input as input image data in a certain cycle and the k-thimage (k is a positive integer), the (k+1)th image, the (k+2)th image,the (k+3)th image, the (k+4)th image, and the (k+5)th image aresequentially displayed at an interval which is 2/5 times the cycle ofthe input image data, the k-th image is displayed in accordance with thei-th image data, the (k+1)th image is displayed in accordance with imagedata corresponding to movement obtained by multiplication of the amountof movement from the i-th image data to the (i+1)th image data by 2/5,the (k+2)th image is displayed in accordance with image datacorresponding to movement obtained by multiplication of the amount ofmovement from the i-th image data to the (i+1)th image data by 4/5, the(k+3)th image is displayed in accordance with image data correspondingto movement obtained by multiplication of the amount of movement fromthe (i+1)th image data to the (i+2)th image data by 1/5, the (k+4)thimage is displayed in accordance with image data corresponding tomovement obtained by multiplication of the amount of movement from the(i+1)th image data to the (i+2)th image data by 3/5, and the (k+5)thimage is displayed in accordance with the (i+2)th image data.

Even specifically, in a driving method of a display device in which,when the conversion ratio is 5/2 (n/m=5/2), the i-th image data (i is apositive integer), the (i+1)th image data, and the (i+2)th image dataare sequentially input as input image data in a certain cycle and thek-th image (k is a positive integer), the (k+1)th image, the (k+2)thimage, the (k+3)th image, the (k+4)th image, and the (k+5)th image aresequentially displayed at an interval which is 2/5 times the cycle ofthe input image data, the k-th image is displayed in accordance with thei-th image data, the (k+1)th image is displayed in accordance with thei-th image data, the (k+2)th image is displayed in accordance with thei-th image data, the (k+3)th image is displayed in accordance with the(i+1)th image data, the (k+4)th image is displayed in accordance withthe (i+1)th image data, and the (k+5)th image is displayed in accordancewith the (i+2)th image data.

When the conversion ratio is 5/2, quality of moving images can beimproved compared with the case where the conversion ratio is less than5/2. Moreover, when the conversion ratio is 5/2, power consumption andmanufacturing cost can be reduced compared with the case where theconversion ratio is more than 5/2.

Specifically, when the conversion ratio is 5/2, driving is also referredto as 5/2-fold frame rate driving or 2.5-fold frame rate driving. Forexample, when the input frame rate is 60 Hz, the display frame rate is150 Hz (150 Hz driving). Accordingly, five images are continuouslydisplayed with respect to two input images. At this time, when aninterpolation image is an intermediate image obtained by motioncompensation, the movement of moving images can be made to be smooth;thus, quality of the moving image can be significantly improved.Moreover, an interpolation image obtained by more accurate motioncompensation can be used, in particular, compared with a driving methodwith low driving frequency, such as 120 Hz driving (double-frame ratedriving) or 180 Hz driving (triple-frame rate driving); thus, themovement of moving images can be made smoother, and quality of themoving image can be significantly improved. Further, operating frequencyof a circuit used for obtaining an intermediate image by motioncompensation can be reduced, in particular, compared with a drivingmethod with high driving frequency, such as 180 Hz driving (triple-framerate driving); thus, an inexpensive circuit can be used, andmanufacturing cost and power consumption can be reduced. Furthermore,when the display device is an active matrix liquid crystal displaydevice, a problem of lack of writing voltage due to dynamic capacitancecan be avoided; thus, quality of moving images can be significantlyimproved, in particular with respect to defects such as an afterimageand a phenomenon of a moving image in which traces are seen. Moreover, acombination of 150 Hz driving and alternating-current driving of aliquid crystal display device is effective. That is, when drivingfrequency of the liquid crystal display device is 150 Hz and frequencyof alternating-current driving is an integer multiple of 150 Hz or aunit fraction of 150 Hz (e.g., 30 Hz, 50 Hz, 75 Hz, or 150 Hz), flickerswhich appear in alternating-current driving can be reduced to a levelthat cannot be perceived by human eyes.

In this manner, by setting positive integers n and m to be a variety ofnumbers, the conversion ratio can be set to be a given rational number(n/m). Although detailed description is omitted, when n is 10 or less,combinations listed below can be possible: n=1 and m=1, that is, theconversion ratio is (n/m)=1 (one-times frame rate driving, 60 Hz), n=2and m=1, that is, the conversion ratio is (n/m)=2 (double-frame ratedriving, 120 Hz), n=3 and m=1, that is, the conversion ratio is (n/m)=3(triple-frame rate driving, 180 Hz), n=3 and m=2, that is, theconversion ratio is (n/m)=3/2 (3/2-fold frame rate driving, 90 Hz), n=4and m=1, that is, the conversion ratio is (n/m)=4 (quadruple-frame ratedriving, 240 Hz), n=4 and m=3, that is, the conversion ratio is(n/m)=4/3 (4/3-fold frame rate driving, 80 Hz), n=5 and m=1, that is,the conversion ratio is (n/m)=5/1 (5-fold frame rate driving, 300 Hz),n=5 and m=2, that is, the conversion ratio is (n/m)=5/2 (5/2-fold framerate driving, 150 Hz), n=5 and m=3, that is, the conversion ratio is(n/m)=5/3 (5/3-fold frame rate driving, 100 Hz), n=5 and m=4, that is,the conversion ratio is (n/m)=5/4 (5/4-fold frame rate driving, 75 Hz),n=6 and m=1, that is, the conversion ratio is (n/m)=6 (6-fold frame ratedriving, 360 Hz), n=6 and m=5, that is, the conversion ratio is(n/m)=6/5 (6/5-fold frame rate driving, 72 Hz), n=7 and m=1, that is,the conversion ratio is (n/m)=7 (7-fold frame rate driving, 420 Hz), n=7and m=2, that is, the conversion ratio is (n/m)=7/2 (7/2-fold frame ratedriving, 210 Hz), n=7 and m=3, that is, the conversion ratio is(n/m)=7/3 (7/3-fold frame rate driving, 140 Hz), n=7 and m=4, that is,the conversion ratio is (n/m)=7/4 (7/4-fold frame rate driving, 105 Hz),n=7 and m=5, that is, the conversion ratio is (n/m)=7/5 (7/5-fold framerate driving, 84 Hz), n=7 and m=6, that is, the conversion ratio is(n/m)=7/6 (7/6-frame rate driving, 70 Hz), n=8 and m=1, that is, theconversion ratio is (n/m)=8 (8-fold frame rate driving, 480 Hz), n=8 andm=3, that is, the conversion ratio is (n/m)=8/3 (8/3-fold frame ratedriving, 160 Hz), n=8 and m=5, that is, the conversion ratio is(n/m)=8/5 (8/5-fold frame rate driving, 96 Hz), n=8 and m=7, that is,the conversion ratio is (n/m)=8/7 (8/7-fold frame rate driving, 68.6Hz), n=9 and m=1, that is, the conversion ratio is (n/m)=9 (9-fold framerate driving, 540 Hz), n=9 and m=2, that is, the conversion ratio is(n/m)=9/2 (9/2-fold frame rate driving, 270 Hz), n=9 and m=4, that is,the conversion ratio is (n/m)=9/4 (9/4-fold frame rate driving, 135 Hz),n=9 and m=5, that is, the conversion ratio is (n/m)=9/5 (9/5-fold framerate driving, 108 Hz), n=9 and m=7, that is, the conversion ratio is(n/m)=9/7 (9/7-frame rate driving, 77.1 Hz), n=9 and m=8, that is, theconversion ratio is (n/m)=9/8 (9/8-fold frame rate driving, 67.5 Hz),n=10 and m=1, that is, the conversion ratio is (n/m)=10 (10-fold framerate driving, 600 Hz), n=10 and m=3, that is, the conversion ratio is(n/m)=10/3 (10/3-fold frame rate driving, 200 Hz), n=10 and m=7, thatis, the conversion ratio is (n/m)=10/7 (10/7-fold frame rate driving,85.7 Hz), and n=10 and m=9, that is, the conversion ratio is (n/m)=10/9(10/9-fold frame rate driving, 66.7 Hz). Note that these frequencies areexamples in the case where the input frame rate is 60 Hz. With regard toother frame rates, the driving frequency is obtained by multiplicationof the each conversion ratio by the input frame rate.

In the case where n is an integer more than 10, although specificnumbers for n and m are not stated here, the procedure of frame rateconversion in the first step can be obviously applied to a variety of nand m.

Depending on how many images which can be displayed without motioncompensation to the input image data are included in images to bedisplayed, the conversion ratio can be determined. Specifically, thesmaller m becomes, the higher the proportion of images which can bedisplayed without motion compensation to the input image data becomes.When motion compensation is performed less frequently, power consumptioncan be reduced because a circuit which performs motion compensationoperates less frequently. Further, the likelihood of generation of animage including an error by motion compensation (an intermediate imagewhich does not correctly reflect motion of an image) can be decreased,so that image quality can be improved. In the case where n is 10 orless, examples of such a conversion ratio include 1, 2, 3, 3/2, 4, 5,5/2, 6, 7, 7/2, 8, 9, 9/2, and 10. By employing such a conversion ratio,especially when an intermediate image obtained by motion compensation isused as an interpolation image, the image quality can be improved andpower consumption can be reduced. This is because the number (half thetotal number of images input) of images which can be displayed withoutmotion compensation to the input image data is comparatively large, andmotion compensation is performed less frequently in the case where m is2; and because the number (equal to the total number of images input) ofimages which can be displayed without motion compensation to the inputimage data is large, and motion compensation cannot be performed in thecase where m is 1. On the other hand, the larger m becomes, the smoothermotion of images can be made because an intermediate image which isgenerated by motion compensation with high accuracy is used.

Note that when a display device is a liquid crystal display device, theconversion ratio can be determined in accordance with a response time ofa liquid crystal element. Here, the response time of the liquid crystalelement is the time from when voltage applied to the liquid crystalelement is changed until when the liquid crystal element responds. Whenthe response time of the liquid crystal element differs depending on theamount of change of the voltage applied to the liquid crystal element,an average of the response times of plural typical voltage changes canbe used. Alternatively, the response time of the liquid crystal elementcan be defined as MRPT (moving picture response time). Then, by framerate conversion, the conversion ratio can be determined so that thelength of the image display cycle can be near the response time of theliquid crystal element. Specifically, the response time of the liquidcrystal element is preferably the time from the value obtained bymultiplication of the cycle of input image data and the inverse numberof the conversion ratio, to approximately half that value. In thismanner, the image display cycle can be made to correspond to theresponse time of the liquid crystal element, so that the image qualityis improved. For example, when the response time of the liquid crystalelement is more than or equal to 4 milliseconds and less than or equalto 8 milliseconds, double-frame rate driving (120 Hz driving) can beemployed. This is because the image display cycle of 120 Hz driving isapproximately 8 milliseconds, and the half of the image display cycle of120 Hz driving is approximately 4 milliseconds. Similarly, for example,when the response time of the liquid crystal element is more than orequal to 3 milliseconds and less than or equal to 6 milliseconds,triple-frame rate driving (180 Hz driving) can be employed; when theresponse time of the liquid crystal element is more than or equal to 5milliseconds and less than or equal to 11 milliseconds, 1.5-fold framerate driving (90 Hz driving) can be employed; when the response time ofthe liquid crystal element is more than or equal to 2 milliseconds andless than or equal to 4 milliseconds, quadruple-frame rate driving (240Hz driving) can be employed; and when the response time of the liquidcrystal element is more than or equal to 6 milliseconds and less than orequal to 12 milliseconds, 1.25-fold frame rate driving (80 Hz driving)can be employed. Note that this is similar to the case of other drivingfrequencies.

Note that the conversion ratio can also be determined by a tradeoffbetween the quality of the moving image, and power consumption andmanufacturing cost. That is, the quality of the moving image can beimproved by increasing the conversion ratio while power consumption andmanufacturing cost can be reduced by decreasing the conversion ratio.Therefore, when n is 10 or less, each conversion ratio has an advantagedescribed below.

When the conversion ratio is 1, the quality of the moving image can bemore improved compared with the case where the conversion ratio is lessthan 1, and power consumption and manufacturing cost can be more reducedcompared with the case where the conversion ratio is more than 1.Moreover, since m is small, power consumption can be reduced while highimage quality is obtained. Further, by applying the conversion ratio of1 to a liquid crystal display device in which the response time of theliquid crystal elements is approximately the same as the cycle of inputimage data, the image quality can be improved.

When the conversion ratio is 2, the quality of the moving image can bemore improved compared with the case where the conversion ratio is lessthan 2, and power consumption and manufacturing cost can be more reducedcompared with the case where the conversion ratio is more than 2.Moreover, since m is small, power consumption can be reduced while highimage quality is obtained. Further, by applying the conversion ratio of2 to a liquid crystal display device in which the response time of theliquid crystal elements is approximately 1/2 times the cycle of inputimage data, the image quality can be improved.

When the conversion ratio is 3, the quality of the moving image can bemore improved compared with the case where the conversion ratio is lessthan 3, and power consumption and manufacturing cost can be more reducedcompared with the case where the conversion ratio is more than 3.Moreover, since m is small, power consumption can be reduced while highimage quality is obtained. Further, by applying the conversion ratio of3 to a liquid crystal display device in which the response time of theliquid crystal elements is approximately 1/3 times the cycle of inputimage data, the image quality can be improved.

When the conversion ratio is 3/2, the quality of the moving image can bemore improved compared with the case where the conversion ratio is lessthan 3/2, and power consumption and manufacturing cost can be morereduced compared with the case where the conversion ratio is more than3/2. Moreover, since m is small, power consumption can be reduced whilehigh image quality is obtained. Further, by applying the conversionratio of 3/2 to a liquid crystal display device in which the responsetime of the liquid crystal elements is approximately 2/3 times the cycleof input image data, the image quality can be improved.

When the conversion ratio is 4, the quality of the moving image can bemore improved compared with the case where the conversion ratio is lessthan 4, and power consumption and manufacturing cost can be more reducedcompared with the case where the conversion ratio is more than 4.Moreover, since m is small, power consumption can be reduced while highimage quality is obtained. Further, by applying the conversion ratio of4 to a liquid crystal display device in which the response time of theliquid crystal elements is approximately 1/4 times the cycle of inputimage data, the image quality can be improved.

When the conversion ratio is 4/3, the quality of the moving image can bemore improved compared with the case where the conversion ratio is lessthan 4/3, and power consumption and manufacturing cost can be morereduced compared with the case where the conversion ratio is more than4/3. Moreover, since m is large, motion of the image can be madesmoother. Further, by applying the conversion ratio of 4/3 to a liquidcrystal display device in which the response time of the liquid crystalelements is approximately 3/4 times the cycle of input image data, theimage quality can be improved.

When the conversion ratio is 5, the quality of the moving image can bemore improved compared with the case where the conversion ratio is lessthan 5, and power consumption and manufacturing cost can be more reducedcompared with the case where the conversion ratio is more than 5.Moreover, since m is small, power consumption can be reduced while highimage quality is obtained. Further, by applying the conversion ratio of5 to a liquid crystal display device in which the response time of theliquid crystal elements is approximately 1/5 times the cycle of inputimage data, the image quality can be improved.

When the conversion ratio is 5/2, the quality of the moving image can bemore improved compared with the case where the conversion ratio is lessthan 5/2, and power consumption and manufacturing cost can be morereduced compared with the case where the conversion ratio is more than5/2. Moreover, since m is small, power consumption can be reduced whilehigh image quality is obtained. Further, by applying the conversionratio of 5/2 to a liquid crystal display device in which the responsetime of the liquid crystal elements is approximately 2/5 times the cycleof input image data, the image quality can be improved.

When the conversion ratio is 5/3, the quality of the moving image can bemore improved compared with the case where the conversion ratio is lessthan 5/3, and power consumption and manufacturing cost can be morereduced compared with the case where the conversion ratio is more than5/3. Moreover, since m is large, motion of the image can be madesmoother. Further, by applying the conversion ratio of 5/3 to a liquidcrystal display device in which the response time of the liquid crystalelements is approximately 3/5 times the cycle of input image data, theimage quality can be improved.

When the conversion ratio is 5/4, the quality of the moving image can bemore improved compared with the case where the conversion ratio is lessthan 5/4, and power consumption and manufacturing cost can be morereduced compared with the case where the conversion ratio is more than5/4. Moreover, since m is large, motion of the image can be madesmoother. Further, by applying the conversion ratio of 5/4 to a liquidcrystal display device in which the response time of the liquid crystalelements is approximately 4/5 times the cycle of input image data, theimage quality can be improved.

When the conversion ratio is 6, the quality of the moving image can bemore improved compared with the case where the conversion ratio is lessthan 6, and power consumption and manufacturing cost can be more reducedcompared with the case where the conversion ratio is more than 6.Moreover, since m is small, power consumption can be reduced while highimage quality is obtained. Further, by applying the conversion ratio of6 to a liquid crystal display device in which the response time of theliquid crystal elements is approximately 1/6 times the cycle of inputimage data, the image quality can be improved.

When the conversion ratio is 6/5, the quality of the moving image can bemore improved compared with the case where the conversion ratio is lessthan 6/5, and power consumption and manufacturing cost can be morereduced compared with the case where the conversion ratio is more than6/5. Moreover, since m is large, motion of the image can be madesmoother. Further, by applying the conversion ratio of 6/5 to a liquidcrystal display device in which the response time of the liquid crystalelements is approximately 5/6 times the cycle of input image data, theimage quality can be improved.

When the conversion ratio is 7, the quality of the moving image can bemore improved compared with the case where the conversion ratio is lessthan 7, and power consumption and manufacturing cost can be more reducedcompared with the case where the conversion ratio is more than 7.Moreover, since m is small, power consumption can be reduced while highimage quality is obtained. Further, by applying the conversion ratio of7 to a liquid crystal display device in which the response time of theliquid crystal elements is approximately 1/7 times the cycle of inputimage data, the image quality can be improved.

When the conversion ratio is 7/2, the quality of the moving image can bemore improved compared with the case where the conversion ratio is lessthan 7/2, and power consumption and manufacturing cost can be morereduced compared with the case where the conversion ratio is more than7/2. Moreover, since m is small, power consumption can be reduced whilehigh image quality is obtained. Further, by applying the conversionratio of 7/2 to a liquid crystal display device in which the responsetime of the liquid crystal elements is approximately 2/7 times the cycleof input image data, the image quality can be improved.

When the conversion ratio is 7/3, the quality of the moving image can bemore improved compared with the case where the conversion ratio is lessthan 7/3, and power consumption and manufacturing cost can be morereduced compared with the case where the conversion ratio is more than7/3. Moreover, since m is large, motion of the image can be madesmoother. Further, by applying the conversion ratio of 7/3 to a liquidcrystal display device in which the response time of the liquid crystalelements is approximately 3/7 times the cycle of input image data, theimage quality can be improved.

When the conversion ratio is 7/4, the quality of the moving image can bemore improved compared with the case where the conversion ratio is lessthan 7/4, and power consumption and manufacturing cost can be morereduced compared with the case where the conversion ratio is more than7/4. Moreover, since m is large, motion of the image can be madesmoother. Further, by applying the conversion ratio of 7/4 to a liquidcrystal display device in which the response time of the liquid crystalelements is approximately 4/7 times the cycle of input image data, theimage quality can be improved.

When the conversion ratio is 7/5, the quality of the moving image can bemore improved compared with the case where the conversion ratio is lessthan 7/5, and power consumption and manufacturing cost can be morereduced compared with the case where the conversion ratio is more than7/5. Moreover, since m is large, motion of the image can be madesmoother. Further, by applying the conversion ratio of 7/5 to a liquidcrystal display device in which the response time of the liquid crystalelements is approximately 5/7 times the cycle of input image data, theimage quality can be improved.

When the conversion ratio is 7/6, the quality of the moving image can bemore improved compared with the case where the conversion ratio is lessthan 7/6, and power consumption and manufacturing cost can be morereduced compared with the case where the conversion ratio is more than7/6. Moreover, since m is large, motion of the image can be madesmoother. Further, by applying the conversion ratio of 7/6 to a liquidcrystal display device in which the response time of the liquid crystalelements is approximately 6/7 times the cycle of input image data, theimage quality can be improved.

When the conversion ratio is 8, the quality of the moving image can bemore improved compared with the case where the conversion ratio is lessthan 8, and power consumption and manufacturing cost can be more reducedcompared with the case where the conversion ratio is more than 8.Moreover, since m is small, power consumption can be reduced while highimage quality is obtained. Further, by applying the conversion ratio of8 to a liquid crystal display device in which the response time of theliquid crystal elements is approximately 1/8 times the cycle of inputimage data, the image quality can be improved.

When the conversion ratio is 8/3, the quality of the moving image can bemore improved compared with the case where the conversion ratio is lessthan 8/3, and power consumption and manufacturing cost can be morereduced compared with the case where the conversion ratio is more than8/3. Moreover, since m is large, motion of the image can be madesmoother. Further, by applying the conversion ratio of 8/3 to a liquidcrystal display device in which the response time of the liquid crystalelements is approximately 3/8 times the cycle of input image data, theimage quality can be improved.

When the conversion ratio is 8/5, the quality of the moving image can bemore improved compared with the case where the conversion ratio is lessthan 8/5, and power consumption and manufacturing cost can be morereduced compared with the case where the conversion ratio is more than8/5. Moreover, since m is large, motion of the image can be madesmoother. Further, by applying the conversion ratio of 8/5 to a liquidcrystal display device in which the response time of the liquid crystalelements is approximately 5/8 times the cycle of input image data, theimage quality can be improved.

When the conversion ratio is 8/7, the quality of the moving image can bemore improved compared with the case where the conversion ratio is lessthan 8/7, and power consumption and manufacturing cost can be morereduced compared with the case where the conversion ratio is more than8/7. Moreover, since m is large, motion of the image can be madesmoother. Further, by applying the conversion ratio of 8/7 to a liquidcrystal display device in which the response time of the liquid crystalelements is approximately 7/8 times the cycle of input image data, theimage quality can be improved.

When the conversion ratio is 9, the quality of the moving image can bemore improved compared with the case where the conversion ratio is lessthan 9, and power consumption and manufacturing cost can be more reducedcompared with the case where the conversion ratio is more than 9.Moreover, since m is small, power consumption can be reduced while highimage quality is obtained. Further, by applying the conversion ratio of9 to a liquid crystal display device in which the response time of theliquid crystal elements is approximately 1/9 times the cycle of inputimage data, the image quality can be improved.

When the conversion ratio is 9/2, the quality of the moving image can bemore improved compared with the case where the conversion ratio is lessthan 9/2, and power consumption and manufacturing cost can be morereduced compared with the case where the conversion ratio is more than9/2. Moreover, since m is small, power consumption can be reduced whilehigh image quality is obtained. Further, by applying the conversionratio of 9/2 to a liquid crystal display device in which the responsetime of the liquid crystal elements is approximately 2/9 times the cycleof input image data, the image quality can be improved.

When the conversion ratio is 9/4, the quality of the moving image can bemore improved compared with the case where the conversion ratio is lessthan 9/4, and power consumption and manufacturing cost can be morereduced compared with the case where the conversion ratio is more than9/4. Moreover, since m is large, motion of the image can be madesmoother. Further, by applying the conversion ratio of 9/4 to a liquidcrystal display device in which the response time of the liquid crystalelements is approximately 4/9 times the cycle of input image data, theimage quality can be improved.

When the conversion ratio is 9/5, the quality of the moving image can bemore improved compared with the case where the conversion ratio is lessthan 9/5, and power consumption and manufacturing cost can be morereduced compared with the case where the conversion ratio is more than9/5. Moreover, since m is large, motion of the image can be madesmoother. Further, by applying the conversion ratio of 9/5 to a liquidcrystal display device in which the response time of the liquid crystalelements is approximately 5/9 times the cycle of input image data, theimage quality can be improved.

When the conversion ratio is 9/7, the quality of the moving image can bemore improved compared with the case where the conversion ratio is lessthan 9/7, and power consumption and manufacturing cost can be morereduced compared with the case where the conversion ratio is more than9/7. Moreover, since m is large, motion of the image can be madesmoother. Further, by applying the conversion ratio of 9/7 to a liquidcrystal display device in which the response time of the liquid crystalelements is approximately 7/9 times the cycle of input image data, theimage quality can be improved.

When the conversion ratio is 9/8, the quality of the moving image can bemore improved compared with the case where the conversion ratio is lessthan 9/8, and power consumption and manufacturing cost can be morereduced compared with the case where the conversion ratio is more than9/8. Moreover, since m is large, motion of the image can be madesmoother. Further, by applying the conversion ratio of 9/8 to a liquidcrystal display device in which the response time of the liquid crystalelements is approximately 8/9 times the cycle of input image data, theimage quality can be improved.

When the conversion ratio is 10, the quality of the moving image can bemore improved compared with the case where the conversion ratio is lessthan 10, and power consumption and manufacturing cost can be morereduced compared with the case where the conversion ratio is more than10. Moreover, since m is small, power consumption can be reduced whilehigh image quality is obtained. Further, by applying the conversionratio of 10 to a liquid crystal display device in which the responsetime of the liquid crystal elements is approximately 1/10 times thecycle of input image data, the image quality can be improved.

When the conversion ratio is 10/3, the quality of the moving image canbe more improved compared with the case where the conversion ratio isless than 10/3, and power consumption and manufacturing cost can be morereduced compared with the case where the conversion ratio is more than10/3. Moreover, since m is large, motion of the image can be madesmoother. Further, by applying the conversion ratio of 10/3 to a liquidcrystal display device in which the response time of the liquid crystalelements is approximately 3/10 times the cycle of input image data, theimage quality can be improved.

When the conversion ratio is 10/7, the quality of the moving image canbe more improved compared with the case where the conversion ratio isless than 10/7, and power consumption and manufacturing cost can be morereduced compared with the case where the conversion ratio is more than10/7. Moreover, since m is large, motion of the image can be madesmoother. Further, by applying the conversion ratio of 10/7 to a liquidcrystal display device in which the response time of the liquid crystalelements is approximately 7/10 times the cycle of input image data, theimage quality can be improved.

When the conversion ratio is 10/9, the quality of the moving image canbe more improved compared with the case where the conversion ratio isless than 10/9, and power consumption and manufacturing cost can be morereduced compared with the case where the conversion ratio is more than10/9. Moreover, since m is large, motion of the image can be madesmoother. Further, by applying the conversion ratio of 10/9 to a liquidcrystal display device in which the response time of the liquid crystalelements is approximately 9/10 times the cycle of input image data, theimage quality can be improved.

Note that it is obvious that each conversion ratio where n is more than10 also has a similar advantage.

Next, as the second step, a method is described in which a plurality ofdifferent images (sub-images) are generated from an image based on inputimage data or each image (hereinafter referred to as an original image)whose frame rate is converted by a given rational number (n/m) times inthe first step, and the plurality of sub-images are displayed intemporal succession. In this manner, human eyes can be made to perceivedisplay such that one original image appears to be displayed, despitethe fact that a plurality of different images are displayed.

Here, among the sub-images generated from one original image, asub-image which is displayed first is referred to as a first sub-image.The timing when the first sub-image is displayed is the same as thetiming when the original image determined in the first step isdisplayed. On the other hand, a sub-image which is displayed thereafteris referred to as a second sub-image. The timing when the secondsub-image is displayed can be optionally determined regardless of thetiming when the original image determined in the first step isdisplayed. Note that an image which is actually displayed is an imagegenerated from the original image by a method in the second step. Avariety of images can be used as the original image for generatingsub-images. The number of sub-images is not limited to two, and morethan two sub-images are also possible. In the second step, the number ofsub-images is represented as J (J is an integer of 2 or more). At thattime, a sub-image which is displayed at the same timing as the timingwhen the original image determined in the first step is displayed isreferred to as a first sub-image. Sub-images which are sequentiallydisplayed are referred to as a second sub-image, a third sub image . . .and a J-th sub-image in order of display.

There are a variety of methods for generating a plurality of sub-imagesfrom one original image. Typically, the following methods can be given.The first method is a method in which the original image is used as itis as the sub-image. The second method is a method in which brightnessof the original image is divided between the plurality of sub-images.The third method is a method in which an intermediate image obtained bymotion compensation is used as the sub-image.

Here, a method of dividing brightness of the original image between theplurality of sub-images can be further divided into some methods.Typically, the following methods can be given. The first method is amethod in which at least one sub-image is a black image (hereinafterreferred to as a black insertion method). The second method is a methodin which the brightness of the original image is divided between aplurality of ranges and just one sub-image among all the sub-images isused to control the brightness in the ranges (hereinafter referred to asa time-division gray scale control method). The third method is a methodin which one sub-image is a bright image which is made by changing agamma value of the original image, and the other sub-image is a darkimage which is made by changing the gamma value of the original image(hereinafter referred to as a gamma correction method).

Some of the methods described above will be briefly described. In themethod in which the original image is used as it is as the sub-image,the original image is used as it is as the first sub-image. Further, theoriginal image is used as it is as the second sub-image. By using thismethod, a circuit which newly generates a sub-image does not need tooperate, or the circuit itself is not necessary; thus, power consumptionand manufacturing cost can be reduced. Particularly in a liquid crystaldisplay device, this method is preferably used after frame rateconversion using an intermediate image obtained by motion compensationin the first step as an interpolation image. This is because when theintermediate image obtained by motion compensation is used as theinterpolation image to make motion of the moving image smooth and thesame image is displayed repeatedly, defects such as afterimages and aphenomenon that phenomenon of a moving image in which traces are seenattributed to lack of writing voltage due to dynamic capacitance of theliquid crystal elements can be reduced.

Next, in the method in which the brightness of the original image isdivided between the plurality of sub-images, a method of setting thebrightness of the image and the length of a period when the sub-imagesare displayed will be described in detail. Note that J is the number ofsub-images, and an integer of 2 or more. The small letter j and thecapital letter J are distinguished. The small letter j is an integer ofmore than or equal to 1 and less than or equal to J. When the brightnessof a pixel in normal hold driving is denote by L, the cycle of originalimage data is denoted by T, the brightness of a pixel in a j-thsub-image is denoted by Lj, and the length of a period when the j-thsub-image is displayed is Tj, it is preferably to set L_(j) and Tj sothat the following formulae are satisfied.

$\begin{matrix}{{LT} = {\sum\limits_{j = 1}^{J}{L_{j}T_{j}}}} & \lbrack {{Formula}\mspace{14mu} 1} \rbrack \\{T = {\sum\limits_{j = 1}^{J}{Tj}}} & \lbrack {{Formula}\mspace{14mu} 2} \rbrack\end{matrix}$

In the methods for dividing brightness of the original image between aplurality of sub-images, a black insertion method is a method in whichat least one sub-image is a black image. In this manner, a displaymethod can be made close to pseudo impulse type display, so thatdeterioration of quality of moving image due to hold-type display methodcan be prevented. In order to prevent decrease in brightness due toblack insertion, conditions of Formulae 1 and 2 are preferablysatisfied. However, in the situation that decrease in brightness of thedisplayed image is acceptable (dark surrounding or the like) or in thecase where decrease in brightness of the displayed image is set to beacceptable by the user, the conditions of Formulae 1 and 2 are notnecessarily satisfied. For example, one sub-image may be the same as theoriginal image, and the other sub-image can be a black image. In thiscase, power consumption can be reduced compared with the case where theconditions of Formulae 1 and 2 are satisfied. Further, in a liquidcrystal display device, when one sub-image is made by increasing thewhole brightness of the original image without limitation of the maximumbrightness, the conditions of Formulae 1 and 2 may be satisfied byincreasing brightness of a backlight. In this case, since the conditionsof Formulae 1 and 2 can be satisfied without controlling the voltagevalue applied to a pixel, operation of an image processing circuit canbe omitted, so that power consumption can be reduced.

Note that a feature of the black insertion method is to make L_(j) ofall pixels 0 in any one of sub-images. In this manner, a display methodcan be made close to pseudo impulse type display, so that deteriorationof quality of moving images due to a hold-type display method can beprevented.

In the methods of dividing the brightness of the original image betweena plurality of sub-images, a time-division gray scale control method isa method in which brightness of the original image is divided into aplurality of ranges and brightness in the range is controlled by justone sub-image among all the sub-images. In this manner, a display methodcan be made close to pseudo impulse type display without decrease inbrightness. Accordingly, deterioration of quality of moving images dueto a hold-type display method can be prevented.

As a method of dividing the brightness of the original image into aplurality of ranges, a method in which the maximum brightness (L_(max))is divided into the number of sub-images can be given. This method willbe described with a display device which can adjust brightness of 0 toL_(max) by 256 levels (the gray scale level of 0 to 255) in the casewhere two sub-images are provided. When the gray scale level of 0 to 127is displayed, brightness of one sub-image is adjusted in the range ofthe gray scale level of 0 to 255 while brightness of the other sub-imageis set to be the gray scale level of 0. When the gray scale level of 128to 255 is displayed, the brightness of one sub-image is set to be thegray scale level of 255 while brightness of the other sub-image isadjusted in the range of the gray scale level of 0 to 255. In thismanner, human eyes can be made to perceive display such that an originalimage appears to be displayed, and display can be close to pseudoimpulse type display; thus, deterioration of quality of moving imagesdue to a hold-type display method can be prevented. Note that more thantwo sub-images can be provided. For example, when three sub-images areprovided, the level (the gray scale level of 0 to 255) of brightness ofan original image is divided into three. In some cases, the number oflevels of brightness is not divisible by the number of sub-images,depending on the number of levels of brightness of the original imageand the number of sub-images; however, the number of levels ofbrightness which is included in the range of each divided brightness canbe distributed as appropriate even if the number of levels of brightnessis not just the same as the number of sub-images.

In the case of a time-division gray scale control method, by satisfyingthe conditions of Formulae 1 and 2, the same image as the original imagecan be displayed without decrease in brightness or the like, which ispreferable.

In the methods of dividing brightness of the original image between aplurality of sub-images, a gamma correction method is a method in whichone sub-image is made a bright image by changing gamma characteristic ofthe original image while the other sub-image is made a dark image bychanging the gamma characteristic of the original image. In this manner,a display method can be made close to pseudo impulse type displaywithout decrease in brightness. Accordingly, deterioration of quality ofmoving images due to a hold-type display method can be prevented. Here,a gamma characteristic is a degree of brightness with respect to a level(a gray scale) of brightness. In general, a line of the gammacharacteristic is adjusted so as to be close to a linear shape. This isbecause a smooth gray scale can be obtained when change in brightness isproportional to one gray scale in the level of brightness. In the gammacorrection method, the curve of the gamma characteristic of onesub-image is deviated from the linear shape so that the one sub-image isbrighter than a sub-image in the linear shape in a region ofintermediate brightness (halftone) (the image in halftone is brighterthan as it usually is). Further, a line of the gamma characteristic ofthe other sub-image is also deviated from the linear shape so that theother sub-image is darker than the sub-image in the linear shape in aregion of intermediate brightness (the image in halftone is darker thanas it usually is). Here, the amount of change for brightening the onesub-image than that in the linear shape and the amount of change fordarkening the other sub-image than the sub-image in the linear shape arepreferably almost the same. This method can make human eyes perceive asif an original image is displayed, and decrease in quality of movingimages due to a hold-type display method can be prevented. Note thatmore than two sub-images can be provided. For example, when threesub-images are provided, each gamma characteristic of three sub-imagesis adjusted so that the sum of the amounts of change for brighteningsub-images and the sum of the amounts of change for darkening sub-imagesare almost the same.

Note that also in the case of the gamma correction method, by satisfyingthe conditions of Formulae 1 and 2, the same image as the original imagecan be displayed without decrease in brightness or the like, which ispreferable. Further, in the gamma correction method, since change inbrightness L_(j) of each sub-image with respect to a gray scale followsa gamma curve, the gray scale of each sub-image can be displayedsmoothly by itself. Accordingly, quality of images which are finallyperceived by human eyes can be improved.

A method in which an intermediate image obtained by motion compensationis used as a sub-image is a method in which one sub-image is anintermediate image obtained by motion compensation using previous andnext images. In this manner, motion of images can be smooth, and qualityof moving images can be improved.

A relation between the timing when a sub-image is displayed and a methodof making a sub-image will be described. Although the timing when thefirst sub-image is displayed is the same as that when the original imagedetermined in the first step is displayed, and the timing when thesecond sub-image is displayed can be optionally decided regardless ofthe timing when the original image determined in the first step isdisplayed, the sub-image itself may be changed in accordance with thetiming when the second sub-image is displayed. In this manner, even ifthe timing when the second sub-image is displayed is changed variously,human eyes can be made to perceive as if the original image isdisplayed. Specifically, if the timing when the second sub-image isdisplayed is earlier, the first sub-image can be brighter and the secondsub-image can be darker. Further, if the timing when the secondsub-image is displayed is later, the first sub-image can be darker andthe second sub-image can be brighter. This is because brightnessperceived by human eyes changes in accordance with the length of aperiod when an image is displayed. More specifically, the longer thelength of the period when an image is displayed becomes, the higherbrightness perceived by human eyes becomes, whereas the shorter thelength of the period when an image is displayed becomes, the lowerbrightness perceived by human eyes becomes. That is, by making thetiming when the second sub-image is displayed earlier, the length of theperiod when the first sub-image is displayed becomes shorter and thelength of period when the second sub-image is displayed becomes longer.This means human eyes perceive as if the first sub-image is dark and thesecond sub-image is bright. As a result, a different image from theoriginal image is perceived by human eyes. In order to prevent this, thefirst sub-image can be made much brighter and the second sub-image canbe made much darker. Similarly, in the case where the length of theperiod when the first sub-image is displayed becomes longer and thelength of the period when the second sub-image is displayed becomesshorter by making the timing when the second sub-image is displayedlater, the first sub-image can be made much darker and the secondsub-image can be made much brighter.

In accordance with the above description, procedures in the second stepare shown below. As a procedure 1, a method of making a plurality ofsub-images from one original image is decided. More specifically, amethod of making a plurality of sub-images can be selected from a methodin which an original image is used as it is as a sub-image, a method inwhich brightness of an original image is divided between a plurality ofsub-images, and a method in which an intermediate image obtained bymotion compensation is used as a sub-image. As a procedure 2, the numberJ of sub-images is decided. Note that J is an integer of 2 or more. As aprocedure 3, the brightness L_(j) of a pixel in the j-th sub-image andthe length T_(j) of the period when the j-th sub-image is displayed aredecided in accordance with the method selected in the procedure 1.Through the procedure 3, the length of a period when each sub-image isdisplayed and the brightness of each pixel included in each sub-imageare specifically decided. As a procedure 4, the original image isprocessed in accordance with the decisions in the respective procedures1 to 3 to actually perform display. As a procedure 5, the objectiveoriginal image is shifted to the next original image, and the operationreturns to the procedure 1.

Note that a mechanism for performing the procedures in the second stepmay be mounted on a device or decided in the design phase of the devicein advance. When the mechanism for performing the procedures in thesecond step is mounted on the device, a driving method can be switchedso that optimal operations depending on circumstances can be performed.Note that the circumstances here include contents of image data,environment inside and outside the device (e.g., temperature, humidity,barometric pressure, light, sound, electric field, the amount ofradiation, altitude, acceleration, or movement speed), user settings,software version, and the like. On the other hand, when the mechanismfor performing the procedures in the second step is decided in thedesign phase of the device in advance, driver circuits optimal forrespective driving methods can be used. Moreover, since the mechanism isdecided, manufacturing cost can be reduced due to efficiency of massproduction.

Next, a variety of driving methods which are decided depending on theprocedures in the second step are described in detail by the use ofspecific values of n and m in the first step.

In the procedure 1 in the second step, when a method in which anoriginal image is used as it is as a sub-image is selected, the drivingmethod is as follows.

The i-th image data (i is a positive integer) and the (i+1)th image dataare sequentially prepared in a constant period T. The period T isdivided into J numbers of sub-image display periods (J is an integer of2 or more). The i-th image data can make each of a plurality of pixelshave unique brightness L. The j-th sub-image (j is an integer of 1 to J)is formed by arranging a plurality of pixels each having the uniquebrightness L_(j) and is displayed only during the j-th sub-image displayperiod T_(j). In a driving method of a display device in which theaforementioned L, T, L_(j), and T_(j) satisfy Formulae 1 and 2, thebrightness L_(j) of each pixel which is included in the j-th sub-imageis equal to L in all values of j. As image data which is sequentiallyprepared in a constant period T, original image data formed in the firststep can be used. That is, all the display patterns given in theexplanation for the first step can be combined with the above-describeddriving method.

Then, when the number J of sub-images is decided to be 2 in theprocedure 2 in the second step and T₁=T₂=T/2 is decided in the procedure3, the above-described driving method is as shown in FIG. 14. In FIG.14, the horizontal axis indicates time, and the vertical axis indicatescases for various combinations of n and m used in the first step.

For example, when n=1 and m=1, that is, the conversion ratio (n/m) is 1in the first step, a driving method as shown in the point where n=1 andm=1 in FIG. 14 is performed. At this time, the display frame rate istwice as high as the frame rate of input image data (double-frame ratedriving). Specifically, for example, when the input frame rate is 60 Hz,the display frame rate is 120 Hz (120 Hz driving). Accordingly, twoimages are continuously displayed with respect to one piece of inputimage data. When double-frame rate driving is performed, quality ofmoving images can be improved compared with the case where the framerate is lower than that of double-frame rate driving, and powerconsumption and manufacturing cost can be reduced compared with the casewhere the frame rate is higher than that of double-frame rate driving.Further, in the procedure 1 in the second step, when a method in whichan original image is used as it is as a sub-image is selected, anoperation of a circuit which produces an intermediate image by motioncompensation can be stopped or the circuit itself can be omitted fromthe device, whereby power consumption and manufacturing cost of thedevice can be reduced. Further, when a display device is an activematrix liquid crystal display device, a problem of lack of writingvoltage due to dynamic capacitance can be avoided; thus, quality ofmoving images can be significantly improved, in particular, with respectto defects such as a phenomenon of a moving image in which traces areseen and an afterimage. Moreover, a combination of 120 Hz driving andalternating-current driving of a liquid crystal display device iseffective. That is, when driving frequency of the liquid crystal displaydevice is 120 Hz and frequency of alternating-current driving is aninteger multiple of 120 Hz or a unit fraction of 120 Hz (e.g., 30 Hz, 60Hz, 120 Hz, or 240 Hz), flickers which appear in alternating-currentdriving can be reduced to a level that cannot be perceived by humaneyes. Moreover, when the driving method is applied to a liquid crystaldisplay device in which response time of the liquid crystal element isapproximately half a cycle of input image data, image quality can beimproved.

Further, for example, when n=2 and m=1, that is, the conversion ratio(n/m) is 2 in the first step, a driving method as shown in the pointwhere n=2 and m=1 in FIG. 14 is performed. At this time, the displayframe rate is four times as high as the frame rate of input image data(quadruple-frame rate driving). Specifically, for example, when theinput frame rate is 60 Hz, the display frame rate is 240 Hz (240 Hzdriving). Accordingly, four images are continuously displayed withrespect to one piece of input image data. At this time, when aninterpolated image in the first step is an intermediate image obtainedby motion compensation, movement of moving images can be smooth; thus,quality of moving images can be significantly improved. In the case ofquadruple-frame rate driving, quality of moving images can be improvedcompared with the case where the frame rate is lower than that ofquadruple-frame rate driving, and power consumption and manufacturingcost can be reduced compared with the case where the frame rate ishigher than that of quadruple-frame rate driving. Further, in theprocedure 1 in the second step, when a method in which an original imageis used as it is as a sub-image is selected, an operation of a circuitwhich produces an intermediate image by motion compensation can bestopped or the circuit itself can be omitted from the device, wherebypower consumption and manufacturing cost of the device can be reduced.Further, when a display device is an active matrix liquid crystaldisplay device, a problem of lack of writing voltage due to dynamiccapacitance can be avoided; thus, quality of moving images can besignificantly improved, in particular, with respect to defects such as aphenomenon of a moving image in which traces are seen and an afterimage.Moreover, a combination of 240 Hz driving and alternating-currentdriving of a liquid crystal display device is effective. That is, whendriving frequency of the liquid crystal display device is 240 Hz andfrequency of alternating-current driving is an integer multiple of 240Hz or a unit fraction of 240 Hz (e.g., 30 Hz, 60 Hz, 120 Hz, or 240 Hz),flickers which appear in alternating-current driving can be reduced to alevel that cannot be perceived by human eyes. Moreover, when the drivingmethod is applied to the liquid crystal display device in which responsetime of the liquid crystal element is approximately 1/4 of a cycle ofinput image data, image quality can be improved.

For example, when n=3 and m=1, that is, the conversion ratio (n/m) is 3in the first step, a driving method as shown in the point where n=3 andm=1 in FIG. 14 is performed. At this time, the display frame rate is sixtimes as high as the frame rate of input image data (6-fold frame ratedriving). Specifically, for example, when the input frame rate is 60 Hz,the display frame rate is 360 Hz (360 Hz driving). Accordingly, siximages are continuously displayed with respect to one piece of inputimage data. At this time, when an interpolated image in the first stepis an intermediate image obtained by motion compensation, movement ofmoving images can be smooth; thus, quality of moving images can besignificantly improved. In the case of 6-fold frame rate driving,quality of moving images can be improved compared with the case wherethe frame rate is lower than that of 6-fold frame rate driving, andpower consumption and manufacturing cost can be reduced compared withthe case where the frame rate is higher than that of 6-fold frame ratedriving. Further, in the procedure 1 in the second step, when a methodin which an original image is used as it is as a sub-image is selected,an operation of a circuit which produces an intermediate image by motioncompensation can be stopped or the circuit itself can be omitted fromthe device, whereby power consumption and manufacturing cost of thedevice can be reduced. Further, when a display device is an activematrix liquid crystal display device, a problem of lack of writingvoltage due to dynamic capacitance can be avoided; thus, quality ofmoving images can be significantly improved, in particular, with respectto defects such as a phenomenon of a moving image in which traces areseen and an afterimage. Moreover, a combination of 360 Hz driving andalternating-current driving of a liquid crystal display device iseffective. That is, when driving frequency of the liquid crystal displaydevice is 360 Hz and frequency of alternating-current driving is aninteger multiple of 360 Hz or a unit fraction of 360 Hz (e.g., 30 Hz, 60Hz, 120 Hz, or 180 Hz), flickers which appear in alternating-currentdriving can be reduced to a level that cannot be perceived by humaneyes. Moreover, when the driving method is applied to the liquid crystaldisplay device in which response time of the liquid crystal element isapproximately 1/6 of a cycle of input image data, image quality can beimproved.

For example, when n=3 and m=2, that is, the conversion ratio (n/m) is3/2 in the first step, a driving method as shown in the point where n=3and m=2 in FIG. 14 is performed. At this time, the display frame rate isthree times as high as the frame rate of input image data (triple-framerate driving). Specifically, for example, when the input frame rate is60 Hz, the display frame rate is 180 Hz (180 Hz driving). Accordingly,three images are continuously displayed with respect to one piece ofinput image data. At this time, when an interpolated image in the firststep is an intermediate image obtained by motion compensation, movementof moving images can be smooth; thus, quality of moving images can besignificantly improved. In the case of triple-frame rate driving,quality of moving images can be improved compared with the case wherethe frame rate is lower than that of triple-frame rate driving, andpower consumption and manufacturing cost can be reduced compared withthe case where the frame rate is higher than that of triple-frame ratedriving. Further, in the procedure 1 in the second step, when a methodin which an original image is used as it is as a sub-image is selected,an operation of a circuit which produces an intermediate image by motioncompensation can be stopped or the circuit itself can be omitted fromthe device, whereby power consumption and manufacturing cost of thedevice can be reduced. Further, when a display device is an activematrix liquid crystal display device, a problem of lack of writingvoltage due to dynamic capacitance can be avoided; thus, quality ofmoving images can be significantly improved, in particular, with respectto defects such as a phenomenon of a moving image in which traces areseen and an afterimage. Moreover, a combination of 180 Hz driving andalternating-current driving of a liquid crystal display device iseffective. That is, when driving frequency of the liquid crystal displaydevice is 180 Hz and frequency of alternating-current driving is aninteger multiple of 180 Hz or a unit fraction of 180 Hz (e.g., 30 Hz, 60Hz, 120 Hz, or 180 Hz), flickers which appear in alternating-currentdriving can be reduced to a level that cannot be perceived by humaneyes. Moreover, when the driving method is applied to the liquid crystaldisplay device in which response time of the liquid crystal element isapproximately 1/3 of a cycle of input image data, image quality can beimproved.

For example, when n=4 and m=1, that is, the conversion ratio (n/m) is 4in the first step, a driving method as shown in the point where n=4 andm=1 in FIG. 14 is performed. At this time, the display frame rate iseight times as high as the frame rate of input image data (8-fold framerate driving). Specifically, for example, when the input frame rate is60 Hz, the display frame rate is 480 Hz (480 Hz driving). Accordingly,eight images are continuously displayed with respect to one piece ofinput image data. At this time, when an interpolated image in the firststep is an intermediate image obtained by motion compensation, movementof moving images can be smooth; thus, quality of moving images can besignificantly improved. In the case of 8-fold frame rate driving,quality of moving images can be improved compared with the case wherethe frame rate is lower than that of 8-fold frame rate driving, andpower consumption and manufacturing cost can be reduced compared withthe case where the frame rate is higher than that of 8-fold frame ratedriving. Further, in the procedure 1 in the second step, when a methodin which an original image is used as it is as a sub-image is selected,an operation of a circuit which produces an intermediate image by motioncompensation can be stopped or the circuit itself can be omitted fromthe device, whereby power consumption and manufacturing cost of thedevice can be reduced. Further, when a display device is an activematrix liquid crystal display device, a problem of lack of writingvoltage due to dynamic capacitance can be avoided; thus, quality ofmoving images can be significantly improved, in particular, with respectto defects such as a phenomenon of a moving image in which traces areseen and an afterimage. Moreover, a combination of 480 Hz driving andalternating-current driving of a liquid crystal display device iseffective. That is, when driving frequency of the liquid crystal displaydevice is 480 Hz and frequency of alternating-current driving is aninteger multiple of 480 Hz or a unit fraction of 480 Hz (e.g., 30 Hz, 60Hz, 120 Hz, or 240 Hz), flickers which appear in alternating-currentdriving can be reduced to a level that cannot be perceived by humaneyes. Moreover, image quality can be improved by applying the drivingmethod to the liquid crystal display device in which the response timeof the liquid crystal element is approximately 1/8 of a cycle of inputimage data.

For example, in the first step, when n=4 and m=3, that is, theconversion ratio (n/m) is 4/3, a driving method as shown in the pointwhere n=4 and m=3 in FIG. 14 is performed. At this time, the displayframe rate is 8/3 times as high as the frame rate of input image data(8/3-fold frame rate driving). Specifically, for example, when the inputframe rate is 60 Hz, the display frame rate is 160 Hz (160 Hz driving).Then, eight images are continuously displayed with respect to threepieces of input image data. At this time, when an interpolated image inthe first step is an intermediate image obtained by motion compensation,movement of moving images can be smooth; thus, quality of moving imagescan be significantly improved. In the case of 8/3-fold frame ratedriving, quality of moving images can be improved compared with the casewhere the frame rate is lower than that of 8/3-fold frame rate driving,and power consumption and manufacturing cost can be reduced comparedwith the case where the frame rate is higher than that of 8/3-fold framerate driving. Further, in the procedure 1 in the second step, when amethod in which an original image is used as it is as a sub-image isselected, an operation of a circuit which produces an intermediate imageby motion compensation can be stopped or the circuit itself can beomitted from the device, whereby power consumption and manufacturingcost of the device can be reduced. Further, when a display device is anactive matrix liquid crystal display device, a problem of lack ofwriting voltage due to dynamic capacitance can be avoided; thus, qualityof moving images can be significantly improved, in particular, withrespect to defects such as a phenomenon of a moving image in whichtraces are seen and an afterimage. Moreover, a combination of 160 Hzdriving and alternating-current driving of a liquid crystal displaydevice is effective. That is, when driving frequency of the liquidcrystal display device is 160 Hz and frequency of alternating-currentdriving is an integer multiple of 160 Hz or a unit fraction of 160 Hz(e.g., 40 Hz, 80 Hz, 160 Hz, or 320 Hz), flickers which appear inalternating-current driving can be reduced to a level that cannot beperceived by human eyes. Moreover, when the driving method is applied tothe liquid crystal display device in which response time of the liquidcrystal element is approximately 3/8 of a cycle of input image data,image quality can be improved.

Further, for example, in the first step, when n=5 and m=1, that is, whenthe conversion ratio (n/m) is 5, a driving method as shown in the pointwhere n=5 and m=1 in FIG. 14 is performed. At this time, the displayframe rate is ten times as high as the frame rate of input image data(10-fold frame rate driving). Specifically, for example, when the inputframe rate is 60 Hz, the display frame rate is 600 Hz (600 Hz driving).Accordingly, ten images are continuously displayed with respect to onepiece of input image data. At this time, when an interpolated image inthe first step is an intermediate image obtained by motion compensation,movement of moving images can be smooth; thus, quality of moving imagescan be significantly improved. In the case of 10-fold frame ratedriving, quality of moving images can be improved compared with the casewhere the frame rate is lower than that of 10-fold frame rate driving,and power consumption and manufacturing cost can be reduced comparedwith the case where the frame rate is higher than that of 10-fold framerate driving. Further, in the procedure 1 in the second step, when amethod in which an original image is used as it is as a sub-image isselected, an operation of a circuit which produces an intermediate imageby motion compensation can be stopped or the circuit itself can beomitted from the device, whereby power consumption and manufacturingcost of the device can be reduced. Further, when a display device is anactive matrix liquid crystal display device, a problem of lack ofwriting voltage due to dynamic capacitance can be avoided; thus, qualityof moving images can be significantly improved, in particular, withrespect to defects such as a phenomenon of a moving image in whichtraces are seen and an afterimage. Moreover, a combination of 600 Hzdriving and alternating-current driving of a liquid crystal displaydevice is effective. That is, when driving frequency of the liquidcrystal display device is 600 Hz and frequency of alternating-currentdriving is an integer multiple of 600 Hz or a unit fraction of 600 Hz(e.g., 30 Hz, 60 Hz, 100 Hz, or 120 Hz), flickers which appear inalternating-current driving can be reduced to a level that cannot beperceived by human eyes. Moreover, when the driving method is applied tothe liquid crystal display device in which response time of the liquidcrystal element is approximately 1/10 of a cycle of input image data,image quality can be improved.

Further, for example, in the first step, when n=5 and m=2, that is, theconversion ratio (n/m) is 5/2, a driving method as shown in the pointwhere n=5 and m=2 in FIG. 14 is performed. At this time, the displayframe rate is five times as high as the frame rate of input image data(5-fold frame rate driving). Specifically, for example, when the inputframe rate is 60 Hz, the display frame rate is 300 Hz (300 Hz driving).Accordingly, five images are continuously displayed with respect to onepiece of input image data. At this time, when an interpolated image inthe first step is an intermediate image obtained by motion compensation,movement of moving images can be smooth; thus, quality of moving imagescan be significantly improved. In the case of 5-fold frame rate driving,quality of moving images can be improved compared with the case wherethe frame rate is lower than that of 5-fold frame rate driving, andpower consumption and manufacturing cost can be reduced compared withthe case where the frame rate is higher than that of 5-fold frame ratedriving. Further, in the procedure 1 in the second step, when a methodin which an original image is used as it is as a sub-image is selected,an operation of a circuit which produces an intermediate image by motioncompensation can be stopped or the circuit itself can be omitted fromthe device, whereby power consumption and manufacturing cost of thedevice can be reduced. Further, when a display device is an activematrix liquid crystal display device, a problem of lack of writingvoltage due to dynamic capacitance can be avoided; thus, quality ofmoving images can be significantly improved, in particular, with respectto defects such as a phenomenon of a moving image in which traces areseen and an afterimage. Moreover, a combination of 300 Hz driving andalternating-current driving of a liquid crystal display device iseffective. That is, when driving frequency of the liquid crystal displaydevice is 300 Hz and frequency of alternating-current driving is aninteger multiple of 300 Hz or a unit fraction of 300 Hz (e.g., 30 Hz, 50Hz, 60 Hz, or 100 Hz), flickers which appear in alternating-currentdriving can be reduced to a level that cannot be perceived by humaneyes. Moreover, when the driving method is applied to the liquid crystaldisplay device in which response time of the liquid crystal element isapproximately 1/5 of a cycle of input image data, image quality can beimproved.

As described above, when a method in which an original image is used asit is as a sub-image is selected the procedure 1 in the second step, thenumber of sub-images is determined to be 2 in the procedure 2 in thesecond step, and T₁=T₂=T/2 is decided in the procedure 3 in the secondstep, the display frame rate can be twice that of frame rate conversionusing a conversion ratio determined by the values of n and m in thefirst step; thus, quality of moving images can be further improved.Further, quality of moving images can be improved compared with the casewhere a display frame rate is lower than the display frame rate, andpower consumption and manufacturing cost can be reduced compared withthe case where a display frame rate is higher than the display framerate. Further, in the procedure 1 in the second step, when a method inwhich an original image is used as it is as a sub-image is selected, anoperation of a circuit which produces an intermediate image by motioncompensation can be stopped or the circuit itself can be omitted fromthe device, whereby power consumption and manufacturing cost of thedevice can be reduced. Further, when a display device is an activematrix liquid crystal display device, a problem of lack of writingvoltage due to dynamic capacitance can be avoided; thus, quality ofmoving images can be significantly improved, in particular, with respectto defects such as a phenomenon of a moving image in which traces areseen and an afterimage. Furthermore, when the driving frequency of theliquid crystal display device is increased and the frequency ofalternating-current driving is an integer multiple or a unit fraction ofthe driving frequency, flickers which appear in alternating-currentdriving can be reduced to a level that cannot be perceived by humaneyes. Moreover, when the driving method is applied to the liquid crystaldisplay device in which response time of the liquid crystal element isapproximately 1/(twice the conversion ratio) of a cycle of input imagedata, image quality can be improved.

Note that although detailed description is omitted, it is apparent thatconversion ratios other than those described above have similaradvantages. For example, in the range that n is equal to or less than10, the combinations can be considered as follows other than thosedescribed above: n=5 and m=3, that is, conversion ratio (n/m) of 5/3(10/3-fold frame rate driving, 200 Hz); n=5 and m=4, that is, conversionratio (n/m) of 5/4 (5/2-fold frame rate driving, 150 Hz); n=6 and m=1,that is, conversion ratio (n/m) of 6 (12-fold frame rate driving, 720Hz); n=6 and m=5, that is, conversion ratio (n/m) of 6/5 (12/5-foldframe rate driving, 144 Hz); n=7 and m=1, that is, conversion ratio(n/m) of 7 (14-fold frame rate driving, 840 Hz); n=7 and m=2, that is,conversion ratio (n/m) of 7/2 (7-fold frame rate driving, 420 Hz); n=7and m=3, that is, conversion ratio (n/m) of 7/3 (14/3-fold frame ratedriving, 280 Hz); n=7 and m=4, that is, conversion ratio (n/m) of 7/4(7/2-fold frame rate driving, 210 Hz); n=7 and m=5, that is, conversionratio (n/m) of 7/5 (14/5-fold frame rate driving, 168 Hz); n=7 and m=6,that is, conversion ratio (n/m) of 7/6 (7/3-fold frame rate driving, 140Hz); n=8 and m=1, that is, conversion ratio (n/m) of 8 (16-fold framerate driving, 960 Hz); n=8 and m=3, that is, conversion ratio (n/m) of8/3 (16/3-fold frame rate driving, 320 Hz); n=8 and m=5, that is,conversion ratio (n/m) of 8/5 (16/5-fold frame rate driving, 192 Hz);n=8 and m=7, that is, conversion ratio (n/m) of 8/7 (16/7-fold framerate driving, 137 Hz); n=9 and m=1, that is, conversion ratio (n/m) of 9(18-fold frame rate driving, 1080 Hz); n=9 and m=2, that is, conversionratio (n/m) of 9/2 (9-fold frame rate driving, 540 Hz); n=9 and m=4,that is, conversion ratio (n/m) of 9/4 (9/2-fold frame rate driving, 270Hz); n=9 and m=5, that is, conversion ratio (n/m) of 9/5 (18/5-foldframe rate driving, 216 Hz); n=9 and m=7, that is, conversion ratio(n/m) of 9/7 (18/7-fold frame rate driving, 154 Hz); n=9 and m=8, thatis, conversion ratio (n/m) of 9/8 (9/4-fold frame rate driving, 135 Hz);n=10 and m=1, that is, conversion ratio (n/m) of 10 (20-fold frame ratedriving, 1200 Hz); n=10 and m=3, that is, conversion ratio (n/m) of 10/3(20/3-fold frame rate driving, 400 Hz); n=10 and m=7, that is,conversion ratio (n/m) of 10/7 (20/7-fold frame rate driving, 171 Hz);and n=10 and m=9, that is, conversion ratio (n/m) of 10/9 (20/9-foldframe rate driving, 133 Hz). Note that these frequencies are examples inthe case where the input frame rate is 60 Hz. In the case of otherfrequencies, the driving frequency is obtained by multiplication of thedoubled conversion ratio by the input frame rate.

Although specific numbers of n and m in the case where n is an integerlarger than 10 are not shown, it is apparent that the processes in thesecond step can be applied to a variety of combinations of n and m.

Note that in the case where J=2, it is particularly effective that theconversion ratio in the first step is larger than 2. This is becausewhen the number of sub-images is comparatively smaller like J=2 in thesecond step, the conversion ratio in the first step can be higher. Whenn is 10 or less, such a conversion ratio includes 3, 4, 5, 5/2, 6, 7,7/2, 7/3, 8, 8/3, 9, 9/2, 9/4, 10, and 10/3. When a display frame rateafter the first step is such a value, by setting the value of J to be 3or more, balance between advantages of the number of sub-images in thesecond step being small (e.g., reduction in power consumption andmanufacturing cost) and advantages of the final display frame rate beinghigh (e.g., increase in quality of moving images and reduction inflickers) can be achieved.

Although this embodiment mode describes the case where the number J ofsub-images is decided to be 2 in the procedure 2 and T₁=T₂=T/2 isdecided in the procedure 3, it is apparent that the invention is notlimited thereto.

For example, when T₁<T₂ is decided in the procedure 3 in the secondstep, the first sub-image can be made brighter and the second sub-imagecan be made darker. Further, when T₁>T₂ is decided in the procedure 3 inthe second step, the first sub-image can be made darker and the secondsub-image can be made brighter. Accordingly, a display method can bepseudo impulse driving, while the original image can be perceived byhuman eyes; thus, quality of moving images can be improved. Note that asin the above-described driving method, when a method in which anoriginal image is used as it is as a sub-image is selected in theprocedure 1, the sub-image can be displayed as it is without changingbrightness of the sub-image. This is because an image used as asub-image is the same in this case, and the original image can bedisplayed properly regardless of display timing of the sub-image.

Further, in the procedure 2, it is obvious that the number J ofsub-images is not limited to 2, and values other than 2 may be employed.In this case, the display frame rate can be J times the frame rate ofthe frame rate conversion of the conversion ratio determined by thevalues of n and m in the first step; thus, quality of moving images canbe further improved. Further, quality of moving images can be improvedcompared with the case where a display frame rate is lower than thedisplay frame rate, and power consumption and manufacturing cost can bereduced compared with the case where a display frame rate is higher thanthe display frame rate. Further, in the procedure 1 in the second step,when a method in which an original image is used as it is as a sub-imageis selected, an operation of a circuit which produces an intermediateimage by motion compensation can be stopped or the circuit itself can beomitted from the device, whereby power consumption and manufacturingcost of the device can be reduced. Further, when a display device is anactive matrix liquid crystal display device, a problem of lack ofwriting voltage due to dynamic capacitance can be avoided; thus, qualityof moving images can be significantly improved, in particular, withrespect to defects such as a phenomenon of a moving image in whichtraces are seen and an afterimage. Furthermore, when the drivingfrequency of the liquid crystal display device is made high and thefrequency of alternating-current driving is an integer multiple or aunit fraction of the driving frequency, flickers which appear inalternating-current driving can be reduced to a level that cannot beperceived by human eyes. Moreover, when the driving method is applied tothe liquid crystal display device in which response time of the liquidcrystal element is approximately 1/(J times of the conversion ratio) ofa cycle of input image data, image quality can be improved.

For example, in the case where J=3, in particular, quality of movingimages can be improved compared with the case where the number ofsub-images is less than 3, and power consumption and manufacturing costcan be reduced compared with the case where the number of sub-images ismore than 3. Moreover, when the driving method is applied to the liquidcrystal display device in which response time of the liquid crystalelement is approximately (1/(three times of the conversion ratio)) of acycle of input image data, image quality can be improved.

For example, in the case where J=4, in particular, quality of movingimages can be improved compared with the case where the number ofsub-images is less than 4, and power consumption and manufacturing costcan be reduced compared with the case where the number of sub-images ismore than 4. Moreover, when the driving method is applied to the liquidcrystal display device in which response time of the liquid crystalelement is approximately 1/(four times of the conversion ratio) of acycle of input image data, image quality can be improved.

For example, in the case where J=5, in particular, quality of movingimages can be improved compared with the case where the number ofsub-images is less than 5, and power consumption and manufacturing costcan be reduced compared with the case where the number of sub-images ismore than 5. Moreover, when the driving method is applied to the liquidcrystal display device in which response time of the liquid crystalelement is approximately 1/(five times of the conversion ratio) of acycle of input image data, image quality can be improved.

Further, J other than those described above has similar advantages.

Note that in the case where J is 3 or more, the conversion ratio in thefirst step can be a variety of values. J of 3 or more is effectiveparticularly when the conversion ratio in the first step is relativelysmall (equal to or less than 2). This is because when a display framerate after the first step is relatively low, J can be larger in thesecond step. When n is 10 or less, such a conversion ratio includes 1,2, 3/2, 4/3, 5/3, 5/4, 6/5, 7/4, 7/5, 7/6, 8/7, 9/5, 9/7, 9/8, 10/7, and10/9. FIG. 15 shows the cases where the conversion ratio is 1, 2, 3/2,4/3, 5/3, and 5/4 among the above-described conversion ratios. Asdescribed above, when the display frame rate after the first step is arelatively small value, by setting the value of J to be 3 or more,balance between advantages of the number of sub-images in the first stepbeing small (e.g., reduction in power consumption and manufacturingcost) and advantages of the final display frame rate being high (e.g.,increase in quality of moving image and reduction in flickers) can beachieved.

Next, another example of a driving method determined by the procedure inthe second step is described in detail by the use of specific values ofn and m in the first step.

In the procedure 1 in the second step, when a black insertion method isselected among methods of dividing brightness of the original imagebetween a plurality of sub-images, the driving method is as follows.

The i-th image data (i is a positive integer) and the (i+1)th image dataare sequentially prepared in a constant period T. The period T isdivided into J sub-image display periods (J is an integer of 2 or more).The i-th image data is data which can make each of a plurality of pixelshave unique brightness L. The j-th sub-image (j is an integer of 1 to J)is formed by arranging a plurality of pixels each having uniquebrightness L_(j) and is displayed only during the j-th sub-image displayperiod T_(j). In a driving method of a display device in which theaforementioned L, T, L_(j), and T_(j) satisfy Formulae 1 and 2, thebrightness L_(j) of all pixels which are included in the j-th sub-imageis equal to 0 in at least one value of j. As image data which issequentially prepared in a constant period T, the original image dataformed in the first step can be used. That is, all the display patternsgiven in the explanation for the first step can be combined with theabove-described driving method.

Then, when the number J of sub-images is decided to be 2 in theprocedure 2 of the second step and T₁=T₂=T/2 is decided in the procedure3, the above-described driving method is as shown in FIG. 14. In FIG.14, the horizontal axis represents time, and the vertical axisrepresents cases for various combinations of n and m used in the firststep.

For example, in the first step, when n=1 and m=1, that is, when theconversion ratio (n/m) is 1, a driving method as shown in the pointwhere n=1 and m=1 in FIG. 14 is performed. At this time, the displayframe rate is twice a frame rate of image data to be input (double-framerate driving). Specifically, for example, when the input frame rate is60 Hz, the display frame rate is 120 Hz (120 Hz driving). Accordingly,two images are continuously displayed with respect to one piece of imagedata to be input. When double-frame rate driving is performed, qualityof moving images can be improved compared with the case where the framerate is lower than that of double-frame rate driving, and powerconsumption and manufacturing cost can be reduced compared with the casewhere the frame rate is higher than that of double-frame rate driving.Further, in the procedure 1 of the second step, when a black insertionmethod is selected among methods where brightness of an original imageis divided between a plurality of sub-images, an operation of a circuitwhich forms an intermediate image by motion compensation can be stoppedor the circuit itself can be omitted from a device; thus, powerconsumption and manufacturing cost of the device can be reduced.Moreover, a pseudo impulse display method can be employed regardless ofa gray scale value included in image data; thus, quality of movingimages can be improved. Further, when the display device is an activematrix liquid crystal display device, a problem of lack of writingvoltage due to dynamic capacitance can be avoided; thus, quality ofmoving images can be significantly improved, in particular with respectto defects such as an afterimage and a phenomenon of a moving image inwhich traces are seen. Furthermore, a combination of 120 Hz driving andalternating-current driving of a liquid crystal display device iseffective. That is, when driving frequency of the liquid crystal displaydevice is 120 Hz and frequency of alternating-current driving is aninteger multiple of 120 Hz or a unit fraction of 120 Hz (e.g., 30 Hz, 60Hz, 120 Hz, or 240 Hz), flickers which appear in alternating-currentdriving can be reduced to a level that cannot be perceived by humaneyes. Moreover, when the driving method is applied to a liquid crystaldisplay device in which response time of a liquid crystal element isapproximately half a cycle of input image data, image quality can beimproved.

For example, in the first step, when n=2 and m=1, that is, when theconversion ratio (n/m) is 2, a driving method as shown in the pointwhere n=2 and m=1 in FIG. 14 is performed. At this time, the displayframe rate is four times a frame rate of image data to be input(quadruple-frame rate driving). Specifically, for example, when theinput frame rate is 60 Hz, the display frame rate is 240 Hz (240 Hzdriving). Accordingly, four images are continuously displayed withrespect to one piece of image data to be input. At this time, when aninterpolation image in the first step is an intermediate image obtainedby motion compensation, the movement of moving images can be made to besmooth; thus, quality of the moving image can be significantly improved.When quadruple-frame rate driving is performed, quality of moving imagescan be improved compared with the case where the frame rate is lowerthan that of quadruple-frame rate driving, and power consumption andmanufacturing cost can be reduced compared with the case where the framerate is higher than that of quadruple-frame rate driving. Further, inthe procedure 1 of the second step, when a black insertion method isselected among methods where brightness of an original image is dividedbetween a plurality of sub-images, an operation of a circuit which formsan intermediate image by motion compensation can be stopped or thecircuit itself can be omitted from a device; thus, power consumption andmanufacturing cost of the device can be reduced. Moreover, a pseudoimpulse display method can be employed regardless of a gray scale valueincluded in image data; thus, quality of moving images can be improved.Further, when the display device is an active matrix liquid crystaldisplay device, a problem of lack of writing voltage due to dynamiccapacitance can be avoided; thus, quality of moving images can besignificantly improved, in particular with respect to defects such as anafterimage and a phenomenon of a moving image in which traces are seen.Furthermore, a combination of 240 Hz driving and alternating-currentdriving of a liquid crystal display device is effective. That is, whendriving frequency of the liquid crystal display device is 240 Hz andfrequency of alternating-current driving is an integer multiple of 240Hz or a unit fraction of 240 Hz (e.g., 30 Hz, 60 Hz, 120 Hz, or 240 Hz),flickers which appear in alternating-current driving can be reduced to alevel that cannot be perceived by human eyes. Moreover, when the drivingmethod is applied to a liquid crystal display device in which responsetime of a liquid crystal element is approximately 1/4 of a cycle ofinput image data, image quality can be improved.

For example, in the first step, when n=3 and m=1, that is, when theconversion ratio (n/m) is 3, a driving method as shown in the pointwhere n=3 and m=1 in FIG. 14 is performed. At this time, the displayframe rate is six times a frame rate of image data to be input (6-foldframe rate driving). Specifically, for example, when the input framerate is 60 Hz, the display frame rate is 360 Hz (360 Hz driving).Accordingly, six images are continuously displayed with respect to onepiece of image data to be input. At this time, when an interpolationimage in the first step is an intermediate image obtained by motioncompensation, the movement of moving images can be made to be smooth;thus, quality of the moving image can be significantly improved. When6-fold frame rate driving is performed, quality of moving images can beimproved compared with the case where the frame rate is lower than thatof 6-fold frame rate driving, and power consumption and manufacturingcost can be reduced compared with the case where the frame rate ishigher than that of 6-fold frame rate driving. Further, in the procedure1 of the second step, when a black insertion method is selected amongmethods where brightness of an original image is divided between aplurality of sub-images, an operation of a circuit which forms anintermediate image by motion compensation can be stopped or the circuititself can be omitted from a device; thus, power consumption andmanufacturing cost of the device can be reduced. Moreover, a pseudoimpulse display method can be employed regardless of a gray scale valueincluded in image data; thus, quality of moving images can be improved.Further, when the display device is an active matrix liquid crystaldisplay device, a problem of lack of writing voltage due to dynamiccapacitance can be avoided; thus, quality of moving images can besignificantly improved, in particular with respect to defects such as anafterimage and a phenomenon of a moving image in which traces are seen.Furthermore, a combination of 360 Hz driving and alternating-currentdriving of a liquid crystal display device is effective. That is, whendriving frequency of the liquid crystal display device is 360 Hz andfrequency of alternating-current driving is an integer multiple of 360Hz or a unit fraction of 360 Hz (e.g., 30 Hz, 60 Hz, 120 Hz, or 180 Hz),flickers which appear in alternating-current driving can be reduced to alevel that cannot be perceived by human eyes. Moreover, when the drivingmethod is applied to a liquid crystal display device in which responsetime of a liquid crystal element is approximately 1/6 of a cycle ofinput image data, image quality can be improved.

For example, in the first step, when n=3 and m=2, that is, when theconversion ratio (n/m) is 3/2, a driving method as shown in the pointwhere n=3 and m=2 in FIG. 14 is performed. At this time, the displayframe rate is three times a frame rate of image data to be input(triple-frame rate driving). Specifically, for example, when the inputframe rate is 60 Hz, the display frame rate is 180 Hz (180 Hz driving).Accordingly, three images are continuously displayed with respect to onepiece of image data to be input. At this time, when an interpolationimage in the first step is an intermediate image obtained by motioncompensation, the movement of moving images can be made to be smooth;thus, quality of the moving image can be significantly improved. Whentriple-frame rate driving is performed, quality of moving images can beimproved compared with the case where the frame rate is lower than thatof triple-frame rate driving, and power consumption and manufacturingcost can be reduced compared with the case where the frame rate ishigher than that of triple-frame rate driving. Further, in the procedure1 of the second step, when a black insertion method is selected amongmethods where brightness of an original image is divided between aplurality of sub-images, an operation of a circuit which forms anintermediate image by motion compensation can be stopped or the circuititself can be omitted from a device; thus, power consumption andmanufacturing cost of the device can be reduced. Moreover, a pseudoimpulse display method can be employed regardless of a gray scale valueincluded in image data; thus, quality of moving images can be improved.Further, when the display device is an active matrix liquid crystaldisplay device, a problem of lack of writing voltage due to dynamiccapacitance can be avoided; thus, quality of moving images can besignificantly improved, in particular with respect to defects such as anafterimage and a phenomenon of a moving image in which traces are seen.Furthermore, a combination of 180 Hz driving and alternating-currentdriving of a liquid crystal display device is effective. That is, whendriving frequency of the liquid crystal display device is 180 Hz andfrequency of alternating-current driving is an integer multiple of 180Hz or a unit fraction of 180 Hz (e.g., 30 Hz, 60 Hz, 120 Hz, or 180 Hz),flickers which appear in alternating-current driving can be reduced to alevel that cannot be perceived by human eyes. Moreover, when the drivingmethod is applied to a liquid crystal display device in which responsetime of a liquid crystal element is approximately 1/3 of a cycle ofinput image data, image quality can be improved.

For example, in the first step, when n=4 and m=1, that is, when theconversion ratio (n/m) is 4, a driving method as shown in the pointwhere n=4 and m=1 in FIG. 14 is performed. At this time, the displayframe rate is eight times a frame rate of image data to be input (8-foldframe rate driving). Specifically, for example, when the input framerate is 60 Hz, the display frame rate is 480 Hz (480 Hz driving).Accordingly, eight images are continuously displayed with respect to onepiece of image data to be input. At this time, when an interpolationimage in the first step is an intermediate image obtained by motioncompensation, the movement of moving images can be made to be smooth;thus, quality of the moving image can be significantly improved. When8-fold frame rate driving is performed, quality of moving images can beimproved compared with the case where the frame rate is lower than thatof 8-fold frame rate driving, and power consumption and manufacturingcost can be reduced compared with the case where the frame rate ishigher than that of 8-fold frame rate driving. Further, in the procedure1 of the second step, when a black insertion method is selected amongmethods where brightness of an original image is divided between aplurality of sub-images, an operation of a circuit which forms anintermediate image by motion compensation can be stopped or the circuititself can be omitted from a device; thus, power consumption andmanufacturing cost of the device can be reduced. Moreover, a pseudoimpulse display method can be employed regardless of a gray scale valueincluded in image data; thus, quality of moving images can be improved.Further, when the display device is an active matrix liquid crystaldisplay device, a problem of lack of writing voltage due to dynamiccapacitance can be avoided; thus, quality of moving images can besignificantly improved, in particular with respect to defects such as anafterimage and a phenomenon of a moving image in which traces are seen.Furthermore, a combination of 480 Hz driving and alternating-currentdriving of a liquid crystal display device is effective. That is, whendriving frequency of the liquid crystal display device is 480 Hz andfrequency of alternating-current driving is an integer multiple of 480Hz or a unit fraction of 480 Hz (e.g., 30 Hz, 60 Hz, 120 Hz, or 240 Hz),flickers which appear in alternating-current driving can be reduced to alevel that cannot be perceived by human eyes. Moreover, when the drivingmethod is applied to a liquid crystal display device in which responsetime of a liquid crystal element is approximately 1/8 of a cycle ofinput image data, image quality can be improved.

For example, in the first step, when n=4 and m=3, that is, when theconversion ratio (n/m) is 4/3, a driving method as shown in the pointwhere n=4 and m=3 in FIG. 14 is performed. At this time, the displayframe rate is 8/3 times a frame rate of image data to be input (8/3-foldframe rate driving). Specifically, for example, when the input framerate is 60 Hz, the display frame rate is 160 Hz (160 Hz driving).Accordingly, eight images are continuously displayed with respect tothree pieces of image data to be input. At this time, when aninterpolation image in the first step is an intermediate image obtainedby motion compensation, the movement of moving images can be made to besmooth; thus, quality of the moving image can be significantly improved.When 8/3-fold frame rate driving is performed, quality of moving imagescan be improved compared with the case where the frame rate is lowerthan that of 8/3-fold frame rate driving, and power consumption andmanufacturing cost can be reduced compared with the case where the framerate is higher than that of 8/3-fold frame rate driving. Further, in theprocedure 1 of the second step, when a black insertion method isselected among methods where brightness of an original image is dividedbetween a plurality of sub-images, an operation of a circuit which formsan intermediate image by motion compensation can be stopped or thecircuit itself can be omitted from a device; thus, power consumption andmanufacturing cost of the device can be reduced. Moreover, a pseudoimpulse display method can be employed regardless of a gray scale valueincluded in image data; thus, quality of moving images can be improved.Further, when the display device is an active matrix liquid crystaldisplay device, a problem of lack of writing voltage due to dynamiccapacitance can be avoided; thus, quality of moving images can besignificantly improved, in particular with respect to defects such as anafterimage and a phenomenon of a moving image in which traces are seen.Furthermore, a combination of 160 Hz driving and alternating-currentdriving of a liquid crystal display device is effective. That is, whendriving frequency of the liquid crystal display device is 160 Hz andfrequency of alternating-current driving is an integer multiple of 160Hz or a unit fraction of 160 Hz (e.g., 40 Hz, 80 Hz, 160 Hz, or 320 Hz),flickers which appear in alternating-current driving can be reduced to alevel that cannot be perceived by human eyes. Moreover, when the drivingmethod is applied to a liquid crystal display device in which responsetime of a liquid crystal element is approximately 3/8 of a cycle ofinput image data, image quality can be improved.

For example, in the first step, when n=5 and m=1, that is, when theconversion ratio (n/m) is 5, a driving method as shown in the pointwhere n=5 and m=1 in FIG. 14 is performed. At this time, the displayframe rate is ten times a frame rate of image data to be input (10-foldframe rate driving). Specifically, for example, when the input framerate is 60 Hz, the display frame rate is 600 Hz (600 Hz driving).Accordingly, ten images are continuously displayed with respect to onepiece of image data to be input. At this time, when an interpolationimage in the first step is an intermediate image obtained by motioncompensation, the movement of moving images can be made to be smooth;thus, quality of the moving image can be significantly improved. When10-fold frame rate driving is performed, quality of moving images can beimproved compared with the case where the frame rate is lower than thatof 10-fold frame rate driving, and power consumption and manufacturingcost can be reduced compared with the case where the frame rate ishigher than that of 10-fold frame rate driving. Further, in theprocedure 1 of the second step, when a black insertion method isselected among methods where brightness of an original image is dividedbetween a plurality of sub-images, an operation of a circuit which formsan intermediate image by motion compensation can be stopped or thecircuit itself can be omitted from a device; thus, power consumption andmanufacturing cost of the device can be reduced. Moreover, a pseudoimpulse display method can be employed regardless of a gray scale valueincluded in image data; thus, quality of moving images can be improved.Further, when the display device is an active matrix liquid crystaldisplay device, a problem of lack of writing voltage due to dynamiccapacitance can be avoided; thus, quality of moving images can besignificantly improved, in particular with respect to defects such as anafterimage and a phenomenon of a moving image in which traces are seen.Furthermore, a combination of 600 Hz driving and alternating-currentdriving of a liquid crystal display device is effective. That is, whendriving frequency of the liquid crystal display device is 600 Hz andfrequency of alternating-current driving is an integer multiple of 600Hz or a unit fraction of 600 Hz (e.g., 30 Hz, 60 Hz, 100 Hz, or 120 Hz),flickers which appear in alternating-current driving can be reduced to alevel that cannot be perceived by human eyes. Moreover, when the drivingmethod is applied to a liquid crystal display device in which responsetime of a liquid crystal element is approximately 1/10 of a cycle ofinput image data, image quality can be improved.

For example, in the first step, when n=5 and m=2, that is, when theconversion ratio (n/m) is 5/2, a driving method as shown in the pointwhere n=5 and m=2 in FIG. 14 is performed. At this time, the displayframe rate is five times a frame rate of image data to be input (5-foldframe rate driving). Specifically, for example, when the input framerate is 60 Hz, the display frame rate is 300 Hz (300 Hz driving).Accordingly, five images are continuously displayed with respect to onepiece of image data to be input. At this time, when an interpolationimage in the first step is an intermediate image obtained by motioncompensation, the movement of moving images can be made to be smooth;thus, quality of the moving image can be significantly improved. When5-fold frame rate driving is performed, quality of moving images can beimproved compared with the case where the frame rate is lower than thatof 5-fold frame rate driving, and power consumption and manufacturingcost can be reduced compared with the case where the frame rate ishigher than that of 5-fold frame rate driving. Further, in the procedure1 of the second step, when a black insertion method is selected amongmethods where brightness of an original image is divided between aplurality of sub-images, an operation of a circuit which forms anintermediate image by motion compensation can be stopped or the circuititself can be omitted from a device; thus, power consumption andmanufacturing cost of the device can be reduced. Moreover, a pseudoimpulse display method can be employed regardless of a gray scale valueincluded in image data; thus, quality of moving images can be improved.Further, when the display device is an active matrix liquid crystaldisplay device, a problem of lack of writing voltage due to dynamiccapacitance can be avoided; thus, quality of moving images can besignificantly improved, in particular with respect to defects such as anafterimage and a phenomenon of a moving image in which traces are seen.Furthermore, a combination of 300 Hz driving and alternating-currentdriving of a liquid crystal display device is effective. That is, whendriving frequency of the liquid crystal display device is 300 Hz andfrequency of alternating-current driving is an integer multiple of 300Hz or a unit fraction of 300 Hz (e.g., 30 Hz, 50 Hz, 60 Hz, or 100 Hz),flickers which appear in alternating-current driving can be reduced to alevel that cannot be perceived by human eyes. Moreover, when the drivingmethod is applied to a liquid crystal display device in which responsetime of a liquid crystal element is approximately 1/5 of a cycle ofinput image data, image quality can be improved.

As described above, when a black insertion method is selected amongmethods where brightness of an original image is divided between aplurality of sub-images in the procedure 1 of the second step, thenumber of sub-images is decided to be 2 in the procedure 2 of the secondstep, and T₁=T₂=T/2 is decided in the procedure 3 of the second step,the frame rate can be twice that of frame rate conversion of theconversion ratio decided by values of n and m in the first step; thus,quality of moving images can be further improved. Further, quality ofmoving images can be improved compared with the case where a displayframe rate is lower than the display frame rate, and power consumptionand manufacturing cost can be reduced compared with the case where adisplay frame rate is higher than the display frame rate. Further, inthe procedure 1 of the second step, when a black insertion method isselected among methods where brightness of an original image is dividedbetween a plurality of sub-images, an operation of the circuit whichforms an intermediate image by motion compensation can be stopped or thecircuit itself can be omitted from the device; thus, power consumptionand manufacturing cost of the device can be reduced. Moreover, a pseudoimpulse display method can be employed regardless of a gray scale valueincluded in image data; thus, quality of moving images can be improved.Further, when the display device is an active matrix liquid crystaldisplay device, a problem of lack of writing voltage due to dynamiccapacitance can be avoided; thus, quality of moving images can besignificantly improved, in particular with respect to defects such as anafterimage and a phenomenon of a moving image in which traces are seen.Furthermore, when driving frequency of the liquid crystal display deviceis increased and frequency of alternating-current driving is an integermultiple of the driving frequency or a unit fraction of the drivingfrequency, flickers which appear in alternating-current driving can bereduced to a level that cannot be perceived by human eyes. Moreover,when the driving method is applied to a liquid crystal display device inwhich response time of a liquid crystal element is approximately1/(twice the conversion ratio) of a cycle of input image data, imagequality can be improved.

Note that although detailed description is omitted, it is apparent thatconversion ratios other than those described above have similaradvantages. For example, in the range that n is equal to or less than10, the combinations can be considered as follows other than thosedescribed above: n=5 and m=3, that is, conversion ratio (n/m) of 5/3(10/3-fold frame rate driving, 200 Hz); n=5 and m=4, that is, conversionratio (n/m) of 5/4 (5/2-fold frame rate driving, 150 Hz); n=6 and m=1,that is, conversion ratio (n/m) of 6 (12-fold frame rate driving, 720Hz); n=6 and m=5, that is, conversion ratio (n/m) of 6/5 (12/5-foldframe rate driving, 144 Hz); n=7 and m=1, that is, conversion ratio(n/m) of 7 (14-fold frame rate driving, 840 Hz); n=7 and m=2, that is,conversion ratio (n/m) of 7/2 (7-fold frame rate driving, 420 Hz); n=7and m=3, that is, conversion ratio (n/m) of 7/3 (14/3-fold frame ratedriving, 280 Hz); n=7 and m=4, that is, conversion ratio (n/m) of 7/4(7/2-fold frame rate driving, 210 Hz); n=7 and m=5, that is, conversionratio (n/m) of 7/5 (14/5-fold frame rate driving, 168 Hz); n=7 and m=6,that is, conversion ratio (n/m) of 7/6 (7/3-fold frame rate driving, 140Hz); n=8 and m=1, that is, conversion ratio (n/m) of 8 (16-fold framerate driving, 960 Hz); n=8 and m=3, that is, conversion ratio (n/m) of8/3 (16/3-fold frame rate driving, 320 Hz); n=8 and m=5, that is,conversion ratio (n/m) of 8/5 (16/5-fold frame rate driving, 192 Hz);n=8 and m=7, that is, conversion ratio (n/m) of 8/7 (16/7-fold framerate driving, 137 Hz); n=9 and m=1, that is, conversion ratio (n/m) of 9(18-fold frame rate driving, 1080 Hz); n=9 and m=2, that is, conversionratio (n/m) of 9/2 (9-fold frame rate driving, 540 Hz); n=9 and m=4,that is, conversion ratio (n/m) of 9/4 (9/2-fold frame rate driving, 270Hz); n=9 and m=5, that is, conversion ratio (n/m) of 9/5 (18/5-foldframe rate driving, 216 Hz); n=9 and m=7, that is, conversion ratio(n/m) of 9/7 (18/7-fold frame rate driving, 154 Hz); n=9 and m=8, thatis, conversion ratio (n/m) of 9/8 (9/4-fold frame rate driving, 135 Hz);n=10 and m=1, that is, conversion ratio (n/m) of 10 (20-fold frame ratedriving, 1200 Hz); n=10 and m=3, that is, conversion ratio (n/m) of 10/3(20/3-fold frame rate driving, 400 Hz); n=10 and m=7, that is,conversion ratio (n/m) of 10/7 (20/7-fold frame rate driving, 171 Hz);and n=10 and m=9, that is, conversion ratio (n/m) of 10/9 (20/9-foldframe rate driving, 133 Hz). Note that these frequencies are examples inthe case where the input frame rate is 60 Hz. In the case of otherfrequencies, the driving frequency is obtained by multiplication of thedoubled conversion ratio by the input frame rate.

Although specific numbers of n and m in the case where n is an integerlarger than 10 are not shown, it is apparent that the procedures in thesecond step can be applied to a variety of combinations of n and m.

Although this embodiment mode describes the case where the number J ofsub-images is decided to be 2 in the procedure 2 and T₁=T₂=T/2 isdecided in the procedure 3, it is apparent that the invention is notlimited thereto.

For example, when T₁<T₂ is decided in the procedure 3 of the secondstep, the first sub-image can be made brighter and the second sub-imagecan be made darker. Further, when T₁>T₂ is decided in the procedure 3 ofthe second step, the first sub-image can be made darker and the secondsub-image can be made brighter. Accordingly, an original image can bewell perceived by human eyes, and at the same time, display can beperformed by pseudo impulse driving; thus, quality of moving images canbe improved. Note that as in the above-described driving method, whenthe black insertion method is selected among methods where brightness ofan original image is divided between a plurality of sub-images, displaycan be performed as it is without changing brightness of the sub-images.This is because when brightness of the sub-images is not changed, thewhole original image is merely displayed with its reduced brightness.That is, when this method is positively used to control brightness of adisplay device, the brightness can be controlled while quality of movingimages is improved.

In addition, in the procedure 2, it is apparent that the number J ofsub-images is not limited to 2, and values other than 2 may be employed.In this case, the display frame rate can be J times the frame rate offrame rate conversion of the conversion ratio determined by the valuesof n and m in the first step; thus, quality of moving images can befurther improved. Further, quality of moving images can be improvedcompared with the case where a display frame rate is lower than thedisplay frame rate, and power consumption and manufacturing cost can bereduced compared with the case where a display frame rate is higher thanthe display frame rate. Further, in the procedure 1 of the second step,when a black insertion method is selected among methods where brightnessof an original image is divided between a plurality of sub-images, anoperation of the circuit which forms an intermediate image by motioncompensation can be stopped or the circuit itself can be omitted fromthe device; thus, power consumption and manufacturing cost of the devicecan be reduced. Moreover, a pseudo impulse display method can beemployed regardless of a gray scale value included in image data; thus,quality of moving images can be improved. Further, when the displaydevice is an active matrix liquid crystal display device, a problem oflack of writing voltage due to dynamic capacitance can be avoided; thus,quality of moving images can be significantly improved, in particularwith respect to defects such as an afterimage and a phenomenon of amoving image in which traces are seen. Furthermore, when drivingfrequency of the liquid crystal display device is increased andfrequency of alternating-current driving is an integer multiple of thedriving frequency or a unit fraction of the driving frequency, flickerswhich appear in alternating-current driving can be reduced to a levelthat cannot be perceived by human eyes. Moreover, when the drivingmethod is applied to a liquid crystal display device in which responsetime of a liquid crystal element is approximately 1/(J times of theconversion ratio) of a cycle of input image data, image quality can beimproved.

For example, in the case where J=3, in particular, quality of movingimages can be improved compared with the case where the number ofsub-images is less than 3. Moreover, power consumption and manufacturingcost can be reduced compared with the case where the number ofsub-images is more than 3. Further, when the driving method is appliedto a liquid crystal display device in which response time of a liquidcrystal element is approximately 1/(three times of the conversion ratio)of a cycle of input image data, image quality can be improved.

For example, in the case where J=4, in particular, quality of movingimages can be improved compared with the case where the number ofsub-images is less than 4. Moreover, power consumption and manufacturingcost can be reduced compared with the case where the number ofsub-images is more than 4. Further, when the driving method is appliedto a liquid crystal display device in which response time of a liquidcrystal element is approximately 1/(four times of the conversion ratio)of a cycle of input image data, image quality can be improved.

In addition, for example, in the case where J=5, in particular, qualityof moving images can be improved compared with the case where the numberof sub-images is less than 5. Moreover, power consumption andmanufacturing cost can be reduced compared with the case where thenumber of sub-images is more than 5. Further, when the driving method isapplied to a liquid crystal display device in which response time of aliquid crystal element is approximately 1/(five times of the conversionratio) of a cycle of input image data, image quality can be improved.

Further, J other than those described above has similar advantages.

Next, another example of a driving method determined by the proceduresin the second step is described in detail by the use of specific valuesof n and m in the first step.

In the procedure 1 of the second step, when a time division gray scalecontrol method is selected among methods of dividing brightness of anoriginal image between a plurality of sub-images, the driving method isas follows.

The i-th image data (i is a positive integer) and the (i+1)th image dataare sequentially prepared in a certain period T. The period is dividedinto J number of sub-image display periods (J is an integer of 2 ormore). The i-th image data can give respective specific values ofbrightness L to a plurality of pixels. The maximum value of the specificbrightness L is L_(max). The j-th sub-image (j is an integer of 1 to J)is formed by arranging a plurality of pixels each having the specificbrightness L_(j) and is displayed in the j-th sub-image display periodT_(j). In a driving method of a display device in which theaforementioned L, T, L_(j), and T_(j) satisfy Formulae 1 and 2, when thespecific brightness L is displayed, brightness in the range of(j−1)×L_(max)/J to j×L_(max)/J is controlled by control of brightness inonly one sub-image display period among the J number of sub-imagedisplay periods. As the image data which is sequentially prepared in thecertain period T, original image data formed in the first step can beused. That is, all the display patterns given in the explanation for thefirst step can be combined with the above-described driving method.

Then, when the number J of sub-images is decided to be 2 in theprocedure 2 of the second step and T₁=T₂=T/2 is decided in the procedure3, the above-described driving method is as shown in FIG. 14. In FIG.14, the horizontal axis represents time, and the vertical axisrepresents cases for various combinations of n and m used in the firststep.

For example, when n=1 and m=1, that is, the conversion ratio (n/m) is 1in the first step, a driving method as shown in the point where n=1 andm=1 in FIG. 14 is performed. At this time, the display frame rate istwice a frame rate of image data to be input (double-frame ratedriving). Specifically, for example, when the input frame rate is 60 Hz,the display frame rate is 120 Hz (120 Hz driving). Accordingly, twoimages are continuously displayed with respect to one piece of imagedata to be input. When double-frame rate driving is performed, qualityof moving images can be improved compared with the case where the framerate is lower than that of double-frame rate driving, and powerconsumption and manufacturing cost can be reduced compared with the casewhere the frame rate is higher than that of double-frame rate driving.Further, in the procedure 1 of the second step, when a time divisiongray scale control method is selected among methods where brightness ofan original image is divided between a plurality of sub-images, anoperation of a circuit which forms an intermediate image by motioncompensation can be stopped or the circuit itself can be omitted from adevice; thus, power consumption and manufacturing cost of the device canbe reduced. Moreover, a pseudo impulse display method can be employed,so that quality of moving images can be improved. Furthermore, sincebrightness of the display device is not reduced, power consumption canbe further reduced. When the display device is an active matrix liquidcrystal display device, a problem of lack of writing voltage due todynamic capacitance can be avoided; thus, quality of moving images canbe significantly improved, in particular with respect to defects such asan afterimage and a phenomenon of a moving image in which traces areseen. Furthermore, a combination of 120 Hz driving andalternating-current driving of a liquid crystal display device iseffective. That is, when driving frequency of the liquid crystal displaydevice is 120 Hz and frequency of alternating-current driving is aninteger multiple of 120 Hz or a unit fraction of 120 Hz (e.g., 30 Hz, 60Hz, 120 Hz, or 240 Hz), flickers which appear in alternating-currentdriving can be reduced to a level that cannot be perceived by humaneyes. Moreover, when the driving method is applied to a liquid crystaldisplay device in which response time of a liquid crystal element isapproximately half a cycle of input image data, image quality can beimproved.

For example, when n=2 and m=1, that is, the conversion ratio (n/m) is 2in the first step, a driving method as shown in the point where n=2 andm=1 in FIG. 14 is performed. At this time, the display frame rate isfour times a frame rate of image data to be input (quadruple-frame ratedriving). Specifically, for example, when the input frame rate is 60 Hz,the display frame rate is 240 Hz (240 Hz driving). Accordingly, fourimages are continuously displayed with respect to one piece of imagedata to be input. At this time, when an interpolation image in the firststep is an intermediate image obtained by motion compensation, themovement of moving images can be made to be smooth; thus, quality of themoving image can be significantly improved. When quadruple-frame ratedriving is performed, quality of moving images can be improved comparedwith the case where the frame rate is lower than that of quadruple-framerate driving, and power consumption and manufacturing cost can bereduced compared with the case where the frame rate is higher than thatof quadruple-frame rate driving. Further, in the procedure 1 of thesecond step, when a time division gray scale control method is selectedamong methods where brightness of an original image is divided between aplurality of sub-images, an operation of a circuit which forms anintermediate image by motion compensation can be stopped or the circuititself can be omitted from a device; thus, power consumption andmanufacturing cost of the device can be reduced. Moreover, a pseudoimpulse display method can be employed, so that quality of moving imagescan be improved. Furthermore, since brightness of the display device isnot reduced, power consumption can be further reduced. When the displaydevice is an active matrix liquid crystal display device, a problem oflack of writing voltage due to dynamic capacitance can be avoided; thus,quality of moving images can be significantly improved, in particularwith respect to defects such as an afterimage and a phenomenon of amoving image in which traces are seen. Furthermore, a combination of 240Hz driving and alternating-current driving of a liquid crystal displaydevice is effective. That is, when driving frequency of the liquidcrystal display device is 240 Hz and frequency of alternating-currentdriving is an integer multiple of 240 Hz or a unit fraction of 240 Hz(e.g., 30 Hz, 60 Hz, 120 Hz, or 240 Hz), flickers which appear inalternating-current driving can be reduced to a level that cannot beperceived by human eyes. Moreover, when the driving method is applied toa liquid crystal display device in which response time of a liquidcrystal element is approximately 1/4 of a cycle of input image data,image quality can be improved.

For example, in the first step, when n=3 and m=1, that is, when theconversion ratio (n/m) is 3, a driving method as shown in the pointwhere n=3 and m=1 in FIG. 14 is performed. At this time, the displayframe rate is six times a frame rate of image data to be input (6-foldframe rate driving). Specifically, for example, when the input framerate is 60 Hz, the display frame rate is 360 Hz (360 Hz driving).Accordingly, six images are continuously displayed with respect to onepiece of image data to be input. At this time, when an interpolationimage in the first step is an intermediate image obtained by motioncompensation, the movement of moving images can be made to be smooth;thus, quality of the moving image can be significantly improved. When6-fold frame rate driving is performed, quality of moving images can beimproved compared with the case where the frame rate is lower than thatof 6-fold frame rate driving, and power consumption and manufacturingcost can be reduced compared with the case where the frame rate ishigher than that of 6-fold frame rate driving. Further, in the procedure1 of the second step, when a time division gray scale control method isselected among methods where brightness of an original image is dividedbetween a plurality of sub-images, an operation of a circuit which formsan intermediate image by motion compensation can be stopped or thecircuit itself can be omitted from a device; thus, power consumption andmanufacturing cost of the device can be reduced. Moreover, a pseudoimpulse display method can be employed, so that quality of moving imagescan be improved. Furthermore, since brightness of the display device isnot reduced, power consumption can be further reduced. Further, when thedisplay device is an active matrix liquid crystal display device, aproblem of lack of writing voltage due to dynamic capacitance can beavoided; thus, quality of moving images can be significantly improved,in particular with respect to defects such as an afterimage and aphenomenon of a moving image in which traces are seen. Furthermore, acombination of 360 Hz driving and alternating-current driving of aliquid crystal display device is effective. That is, when drivingfrequency of the liquid crystal display device is 360 Hz and frequencyof alternating-current driving is an integer multiple of 360 Hz or aunit fraction of 360 Hz (e.g., 30 Hz, 60 Hz, 120 Hz, or 180 Hz),flickers which appear in alternating-current driving can be reduced to alevel that cannot be perceived by human eyes. Moreover, when the drivingmethod is applied to a liquid crystal display device in which responsetime of a liquid crystal element is approximately 1/6 of a cycle ofinput image data, image quality can be improved.

For example, in the first step, when n=3 and m=2, that is, when theconversion ratio (n/m) is 3/2, a driving method as shown in the pointwhere n=3 and m=2 in FIG. 14 is performed. At this time, the displayframe rate is three times a frame rate of image data to be input(triple-frame rate driving). Specifically, for example, when the inputframe rate is 60 Hz, the display frame rate is 180 Hz (180 Hz driving).Accordingly, three images are continuously displayed with respect to onepiece of image data to be input. At this time, when an interpolationimage in the first step is an intermediate image obtained by motioncompensation, the movement of moving images can be made to be smooth;thus, quality of the moving image can be significantly improved. Whentriple-frame rate driving is performed, quality of moving images can beimproved compared with the case where the frame rate is lower than thatof triple-frame rate driving, and power consumption and manufacturingcost can be reduced compared with the case where the frame rate ishigher than that of triple-frame rate driving. Further, in the procedure1 of the second step, when a time division gray scale control method isselected among methods where brightness of an original image is dividedbetween a plurality of sub-images, an operation of a circuit which formsan intermediate image by motion compensation can be stopped or thecircuit itself can be omitted from a device; thus, power consumption andmanufacturing cost of the device can be reduced. Moreover, a pseudoimpulse display method can be employed, so that quality of moving imagescan be improved. Furthermore, since brightness of the display device isnot reduced, power consumption can be further reduced. Further, when thedisplay device is an active matrix liquid crystal display device, aproblem of lack of writing voltage due to dynamic capacitance can beavoided; thus, quality of moving images can be significantly improved,in particular with respect to defects such as an afterimage and aphenomenon of a moving image in which traces are seen. Furthermore, acombination of 180 Hz driving and alternating-current driving of aliquid crystal display device is effective. That is, when drivingfrequency of the liquid crystal display device is 180 Hz and frequencyof alternating-current driving is an integer multiple of 180 Hz or aunit fraction of 180 Hz (e.g., 30 Hz, 60 Hz, 120 Hz, or 180 Hz),flickers which appear in alternating-current driving can be reduced to alevel that cannot be perceived by human eyes. Moreover, when the drivingmethod is applied to a liquid crystal display device in which responsetime of a liquid crystal element is approximately 1/3 of a cycle ofinput image data, image quality can be improved.

For example, in the first step, when n=4 and m=1, that is, when theconversion ratio (n/m) is 4, a driving method as shown in the pointwhere n=4 and m=1 in FIG. 14 is performed. At this time, the displayframe rate is eight times a frame rate of image data to be input (8-foldframe rate driving). Specifically, for example, when the input framerate is 60 Hz, the display frame rate is 480 Hz (480 Hz driving).Accordingly, eight images are continuously displayed with respect to onepiece of image data to be input. At this time, when an interpolationimage in the first step is an intermediate image obtained by motioncompensation, the movement of moving images can be made to be smooth;thus, quality of the moving image can be significantly improved. When8-fold frame rate driving is performed, quality of moving images can beimproved compared with the case where the frame rate is lower than thatof 8-fold frame rate driving, and power consumption and manufacturingcost can be reduced compared with the case where the frame rate ishigher than that of 8-fold frame rate driving. Further, in the procedure1 of the second step, when a time division gray scale control method isselected among methods where brightness of an original image is dividedbetween a plurality of sub-images, an operation of a circuit which formsan intermediate image by motion compensation can be stopped or thecircuit itself can be omitted from a device; thus, power consumption andmanufacturing cost of the device can be reduced. Moreover, a pseudoimpulse display method can be employed, so that quality of moving imagescan be improved. Furthermore, since brightness of the display device isnot reduced, power consumption can be further reduced. Further, when thedisplay device is an active matrix liquid crystal display device, aproblem of lack of writing voltage due to dynamic capacitance can beavoided; thus, quality of moving images can be significantly improved,in particular with respect to defects such as an afterimage and aphenomenon of a moving image in which traces are seen. Furthermore, acombination of 480 Hz driving and alternating-current driving of aliquid crystal display device is effective. That is, when drivingfrequency of the liquid crystal display device is 480 Hz and frequencyof alternating-current driving is an integer multiple of 480 Hz or aunit fraction of 480 Hz (e.g., 30 Hz, 60 Hz, 120 Hz, or 240 Hz),flickers which appear in alternating-current driving can be reduced to alevel that cannot be perceived by human eyes. Moreover, when the drivingmethod is applied to a liquid crystal display device in which responsetime of a liquid crystal element is approximately 1/8 of a cycle ofinput image data, image quality can be improved.

For example, in the first step, when n=4 and m=3, that is, when theconversion ratio (n/m) is 4/3, a driving method as shown in the pointwhere n=4 and m=3 in FIG. 14 is performed. At this time, the displayframe rate is 8/3 times a frame rate of image data to be input (8/3-foldframe rate driving). Specifically, for example, when the input framerate is 60 Hz, the display frame rate is 160 Hz (160 Hz driving).Accordingly, eight images are continuously displayed with respect tothree pieces of image data to be input. At this time, when aninterpolation image in the first step is an intermediate image obtainedby motion compensation, the movement of moving images can be made to besmooth; thus, quality of the moving image can be significantly improved.When 8/3-fold frame rate driving is performed, quality of moving imagescan be improved compared with the case where the frame rate is lowerthan that of 8/3-fold frame rate driving, and power consumption andmanufacturing cost can be reduced compared with the case where the framerate is higher than that of 8/3-fold frame rate driving. Further, in theprocedure 1 of the second step, when a time division gray scale controlmethod is selected among methods where brightness of an original imageis divided between a plurality of sub-images, an operation of a circuitwhich forms an intermediate image by motion compensation can be stoppedor the circuit itself can be omitted from a device; thus, powerconsumption and manufacturing cost of the device can be reduced.Moreover, a pseudo impulse display method can be employed, so thatquality of moving images can be improved. Furthermore, since brightnessof the display device is not reduced, power consumption can be furtherreduced. Further, when the display device is an active matrix liquidcrystal display device, a problem of lack of writing voltage due todynamic capacitance can be avoided; thus, quality of moving images canbe significantly improved, in particular with respect to defects such asan afterimage and a phenomenon of a moving image in which traces areseen. Furthermore, a combination of 160 Hz driving andalternating-current driving of a liquid crystal display device iseffective. That is, when driving frequency of the liquid crystal displaydevice is 160 Hz and frequency of alternating-current driving is aninteger multiple of 160 Hz or a unit fraction of 160 Hz (e.g., 40 Hz, 80Hz, 160 Hz, or 320 Hz), flickers which appear in alternating-currentdriving can be reduced to a level that cannot be perceived by humaneyes. Moreover, when the driving method is applied to a liquid crystaldisplay device in which response time of a liquid crystal element isapproximately 3/8 of a cycle of input image data, image quality can beimproved.

For example, in the first step, when n=5 and m=1, that is, when theconversion ratio (n/m) is 5, a driving method as shown in the pointwhere n=5 and m=1 in FIG. 14 is performed. At this time, the displayframe rate is ten times a frame rate of image data to be input (10-foldframe rate driving). Specifically, for example, when the input framerate is 60 Hz, the display frame rate is 600 Hz (600 Hz driving).Accordingly, ten images are continuously displayed with respect to onepiece of image data to be input. At this time, when an interpolationimage in the first step is an intermediate image obtained by motioncompensation, the movement of moving images can be made to be smooth;thus, quality of the moving image can be significantly improved. When10-fold frame rate driving is performed, quality of moving images can beimproved compared with the case where the frame rate is lower than thatof 10-fold frame rate driving, and power consumption and manufacturingcost can be reduced compared with the case where the frame rate ishigher than that of 10-fold frame rate driving. Further, in theprocedure 1 of the second step, when a time division gray scale controlmethod is selected among methods where brightness of an original imageis divided between a plurality of sub-images, an operation of a circuitwhich forms an intermediate image by motion compensation can be stoppedor the circuit itself can be omitted from a device; thus, powerconsumption and manufacturing cost of the device can be reduced.Moreover, a pseudo impulse display method can be employed, so thatquality of moving images can be improved. Furthermore, since brightnessof the display device is not reduced, power consumption can be furtherreduced. Further, when the display device is an active matrix liquidcrystal display device, a problem of lack of writing voltage due todynamic capacitance can be avoided; thus, quality of moving images canbe significantly improved, in particular with respect to defects such asan afterimage and a phenomenon of a moving image in which traces areseen. Furthermore, a combination of 600 Hz driving andalternating-current driving of a liquid crystal display device iseffective. That is, when driving frequency of the liquid crystal displaydevice is 600 Hz and frequency of alternating-current driving is aninteger multiple of 600 Hz or a unit fraction of 600 Hz (e.g., 30 Hz, 60Hz, 100 Hz, or 120 Hz), flickers which appear in alternating-currentdriving can be reduced to a level that cannot be perceived by humaneyes. Moreover, when the driving method is applied to a liquid crystaldisplay device in which response time of a liquid crystal element isapproximately 1/10 of a cycle of input image data, image quality can beimproved.

For example, in the first step, when n=5 and m=2, that is, when theconversion ratio (n/m) is 5/2, a driving method as shown in the pointwhere n=5 and m=2 in FIG. 14 is performed. At this time, the displayframe rate is five times a frame rate of image data to be input (5-foldframe rate driving). Specifically, for example, when the input framerate is 60 Hz, the display frame rate is 300 Hz (300 Hz driving).Accordingly, five images are continuously displayed with respect to onepiece of image data to be input. At this time, when an interpolationimage in the first step is an intermediate image obtained by motioncompensation, the movement of moving images can be made to be smooth;thus, quality of the moving image can be significantly improved. When5-fold frame rate driving is performed, quality of moving images can beimproved compared with the case where the frame rate is lower than thatof 5-fold frame rate driving, and power consumption and manufacturingcost can be reduced compared with the case where the frame rate ishigher than that of 5-fold frame rate driving. Further, in the procedure1 of the second step, when a time division gray scale control method isselected among methods where brightness of an original image is dividedbetween a plurality of sub-images, an operation of a circuit which formsan intermediate image by motion compensation can be stopped or thecircuit itself can be omitted from a device; thus, power consumption andmanufacturing cost of the device can be reduced. Moreover, a pseudoimpulse display method can be employed, so that quality of moving imagescan be improved. Furthermore, since brightness of the display device isnot reduced, power consumption can be further reduced. Further, when thedisplay device is an active matrix liquid crystal display device, aproblem of lack of writing voltage due to dynamic capacitance can beavoided; thus, quality of moving images can be significantly improved,in particular with respect to defects such as an afterimage and aphenomenon of a moving image in which traces are seen. Furthermore, acombination of 300 Hz driving and alternating-current driving of aliquid crystal display device is effective. That is, when drivingfrequency of the liquid crystal display device is 300 Hz and frequencyof alternating-current driving is an integer multiple of 300 Hz or aunit fraction of 300 Hz (e.g., 30 Hz, 50 Hz, 60 Hz, or 100 Hz), flickerswhich appear in alternating-current driving can be reduced to a levelthat cannot be perceived by human eyes. Moreover, when the drivingmethod is applied to a liquid crystal display device in which responsetime of a liquid crystal element is approximately 1/5 of a cycle ofinput image data, image quality can be improved.

As described above, when a time division gray scale control method isselected in the procedure 1 of the second step among methods wherebrightness of an original image is divided between a plurality ofsub-images, the number of sub-images is decided to be 2 in the procedure2 of the second step, and T₁=T₂=T/2 is decided in the procedure 3 of thesecond step, the frame rate can be twice that of frame rate conversionof the conversion ratio decided by values of n and m in the first step;thus, quality of moving images can be further improved. Further, qualityof moving images can be improved compared with the case where a displayframe rate is lower than the display frame rate, and power consumptionand manufacturing cost can be reduced compared with the case where adisplay frame rate is higher than the display frame rate. Further, inthe procedure 1 of the second step, when a time division gray scalemethod is selected among methods where brightness of an original imageis divided between a plurality of sub-images, an operation of thecircuit which forms an intermediate image by motion compensation can bestopped or the circuit itself can be omitted from the device; thus,power consumption and manufacturing cost of the device can be reduced.Moreover, a pseudo impulse display method can be employed, so thatquality of moving images can be improved. Furthermore, since brightnessof the display device is not reduced, power consumption can be furtherreduced. Further, when the display device is an active matrix liquidcrystal display device, a problem of lack of writing voltage due todynamic capacitance can be avoided; thus, quality of moving images canbe significantly improved, in particular with respect to defects such asan afterimage and a phenomenon of a moving image in which traces areseen. Furthermore, when driving frequency of the liquid crystal displaydevice is increased and frequency of alternating-current driving is aninteger multiple of the driving frequency or a unit fraction of thedriving frequency, flickers which appear in alternating-current drivingcan be reduced to a level that cannot be perceived by human eyes.Moreover, when the driving method is applied to a liquid crystal displaydevice in which response time of a liquid crystal element isapproximately 1/(twice the conversion ratio) of a cycle of input imagedata, image quality can be improved.

Note that although detailed description is omitted, it is apparent thatconversion ratios other than those described above have similaradvantages. For example, in the range that n is equal to or less than10, the combinations can be considered as follows other than thosedescribed above: n=5 and m=3, that is, conversion ratio (n/m) of 5/3(10/3-fold frame rate driving, 200 Hz); n=5 and m=4, that is, conversionratio (n/m) of 5/4 (5/2-fold frame rate driving, 150 Hz); n=6 and m=1,that is, conversion ratio (n/m) of 6 (12-fold frame rate driving, 720Hz); n=6 and m=5, that is, conversion ratio (n/m) of 6/5 (12/5-foldframe rate driving, 144 Hz); n=7 and m=1, that is, conversion ratio(n/m) of 7 (14-fold frame rate driving, 840 Hz); n=7 and m=2, that is,conversion ratio (n/m) of 7/2 (7-fold frame rate driving, 420 Hz); n=7and m=3, that is, conversion ratio (n/m) of 7/3 (14/3-fold frame ratedriving, 280 Hz); n=7 and m=4, that is, conversion ratio (n/m) of 7/4(7/2-fold frame rate driving, 210 Hz); n=7 and m=5, that is, conversionratio (n/m) of 7/5 (14/5-fold frame rate driving, 168 Hz); n=7 and m=6,that is, conversion ratio (n/m) of 7/6 (7/3-fold frame rate driving, 140Hz); n=8 and m=1, that is, conversion ratio (n/m) of 8 (16-fold framerate driving, 960 Hz); n=8 and m=3, that is, conversion ratio (n/m) of8/3 (16/3-fold frame rate driving, 320 Hz); n=8 and m=5, that is,conversion ratio (n/m) of 8/5 (16/5-fold frame rate driving, 192 Hz);n=8 and m=7, that is, conversion ratio (n/m) of 8/7 (16/7-fold framerate driving, 137 Hz); n=9 and m=1, that is, conversion ratio (n/m) of 9(18-fold frame rate driving, 1080 Hz); n=9 and m=2, that is, conversionratio (n/m) of 9/2 (9-fold frame rate driving, 540 Hz); n=9 and m=4,that is, conversion ratio (n/m) of 9/4 (9/2-fold frame rate driving, 270Hz); n=9 and m=5, that is, conversion ratio (n/m) of 9/5 (18/5-foldframe rate driving, 216 Hz); n=9 and m=7, that is, conversion ratio(n/m) of 9/7 (18/7-fold frame rate driving, 154 Hz); n=9 and m=8, thatis, conversion ratio (n/m) of 9/8 (9/4-fold frame rate driving, 135 Hz);n=10 and m=1, that is, conversion ratio (n/m) of 10 (20-fold frame ratedriving, 1200 Hz); n=10 and m=3, that is, conversion ratio (n/m) of 10/3(20/3-fold frame rate driving, 400 Hz); n=10 and m=7, that is,conversion ratio (n/m) of 10/7 (20/7-fold frame rate driving, 171 Hz);and n=10 and m=9, that is, conversion ratio (n/m) of 10/9 (20/9-foldframe rate driving, 133 Hz). Note that these frequencies are examples inthe case where the input frame rate is 60 Hz. In the case of otherfrequencies, the driving frequency is obtained by multiplication of thedoubled conversion ratio by the input frame rate.

Although specific numbers of n and m in the case where n is an integerlarger than 10 are not shown, it is apparent that the processes in thesecond step can be applied to a variety of combinations of n and m.

Although this embodiment mode describes the case where the number J ofsub-images is decided to be 2 in the procedure 2 and T₁=T₂=T/2 isdecided in the procedure 3, it is apparent that the invention is notlimited thereto.

For example, when T₁<T₂ is decided in the procedure 3 of the secondstep, the first sub-image can be made brighter and the second sub-imagecan be made darker. Further, when T₁>T₂ is decided in the procedure 3 ofthe second step, the first sub-image can be made darker and the secondsub-image can be made brighter. Accordingly, an original image can bewell perceived by human eyes, and at the same time, display can beperformed by pseudo impulse driving; thus, quality of moving images canbe improved.

In addition, in the procedure 2, it is apparent that the number J ofsub-images is not limited to 2, and values other than 2 may be employed.In this case, the display frame rate can be J times the frame rate offrame rate conversion of the conversion ratio determined by the valuesof n and m in the first step; thus, quality of moving images can befurther improved. Further, quality of moving images can be improvedcompared with the case where a display frame rate is lower than thedisplay frame rate, and power consumption and manufacturing cost can bereduced compared with the case where a display frame rate is higher thanthe display frame rate. Further, in the procedure 1 of the second step,when a time division gray scale control method is selected among methodswhere brightness of an original image is divided between a plurality ofsub-images, an operation of the circuit which forms an intermediateimage by motion compensation can be stopped or the circuit itself can beomitted from the device; thus, power consumption and manufacturing costof the device can be reduced. Moreover, a pseudo impulse display methodcan be employed, so that quality of moving images can be improved.Furthermore, since brightness of the display device is not reduced,power consumption can be further reduced. Further, when the displaydevice is an active matrix liquid crystal display device, a problem oflack of writing voltage due to dynamic capacitance can be avoided; thus,quality of moving images can be significantly improved, in particularwith respect to defects such as an afterimage and a phenomenon of amoving image in which traces are seen. Furthermore, when drivingfrequency of the liquid crystal display device is increased andfrequency of alternating-current driving is an integer multiple of thedriving frequency or a unit fraction of the driving frequency, flickerswhich appear in alternating-current driving can be reduced to a levelthat cannot be perceived by human eyes. Moreover, when the drivingmethod is applied to a liquid crystal display device in which responsetime of a liquid crystal element is approximately 1/(J times of theconversion ratio) of a cycle of input image data, image quality can beimproved.

For example, in the case where J=3, in particular, quality of movingimages can be improved compared with the case where the number ofsub-images is less than 3. Moreover, power consumption and manufacturingcost can be reduced compared with the case where the number ofsub-images is more than 3. Further, when the driving method is appliedto a liquid crystal display device in which response time of a liquidcrystal element is approximately 1/(three times of the conversion ratio)of a cycle of input image data, image quality can be improved.

For example, in the case where J=4, in particular, quality of movingimages can be improved compared with the case where the number ofsub-images is less than 4. Moreover, power consumption and manufacturingcost can be reduced compared with the case where the number ofsub-images is more than 4. Further, when the driving method is appliedto a liquid crystal display device in which response time of a liquidcrystal element is approximately 1/(four times of the conversion ratio)of a cycle of input image data, image quality can be improved.

In addition, for example, in the case where J=5, in particular, qualityof moving images can be improved compared with the case where the numberof sub-images is less than 5. Moreover, power consumption andmanufacturing cost can be reduced compared with the case where thenumber of sub-images is more than 5. Further, when the driving method isapplied to a liquid crystal display device in which response time of aliquid crystal element is approximately 1/(five times of the conversionratio) of a cycle of input image data, image quality can be improved.

Further, J other than those described above has similar advantages.

Next, another example of a driving method determined by the processes inthe second step is described in detail by the use of specific values ofn and m in the first step.

In the procedure 1 of the second step, when a gamma correction method isselected among methods of dividing brightness of an original imagebetween a plurality of sub-images, the driving method is as follows.

The i-th image data (i is a positive integer) and the (i+1)th image dataare sequentially prepared in a certain period T. The period is dividedinto J number of sub-image display periods (J is an integer of 2 ormore). The i-th image data can give respective specific values ofbrightness L to a plurality of pixels. The j-th sub-image (j is aninteger of 1 to J) is formed by arranging a plurality of pixels eachhaving the specific brightness L_(j) and is displayed in the j-thsub-image display period T_(j). In a driving method of a display devicein which L, T, L_(j), and T_(j) satisfy Formulae 1 and 2, in eachsub-image, characteristics of change in brightness with respect to grayscales changes from a linear shape, and the sum of the amount ofbrightness which is deviated to the brighter side and the sum of theamount of brightness which is deviated to the darker side areapproximately the same in all the gray scales. As the image data whichis sequentially prepared in the certain period T, original image dataformed in the first step can be used. That is, all the display patternsgiven in the explanation for the first step can be combined with theabove-described driving method.

Then, when the number J of sub-images is decided to be 2 in theprocedure 2 of the second step and T₁=T₂=T/2 is decided in the procedure3, the above-described driving method is as shown in FIG. 14. In FIG.14, the horizontal axis represents time, and the vertical axisrepresents cases for various combinations of n and m used in the firststep.

For example, when n=1 and m=1, that is, the conversion ratio (n/m) is 1in the first step, a driving method as shown in the point where n=1 andm=1 in FIG. 14 is performed. At this time, the display frame rate istwice a frame rate of image data to be input (double-frame ratedriving). Specifically, for example, when the input frame rate is 60 Hz,the display frame rate is 120 Hz (120 Hz driving). Accordingly, twoimages are continuously displayed with respect to one piece of imagedata to be input. When double-frame rate driving is performed, qualityof moving images can be improved compared with the case where the framerate is lower than that of double-frame rate driving, and powerconsumption and manufacturing cost can be reduced compared with the casewhere the frame rate is higher than that of double-frame rate driving.Further, in the procedure 1 of the second step, when a gamma correctionmethod is selected among methods where brightness of an original imageis divided between a plurality of sub-images, an operation of a circuitwhich forms an intermediate image by motion compensation can be stoppedor the circuit itself can be omitted from a device; thus, powerconsumption and manufacturing cost of the device can be reduced.Moreover, a pseudo impulse display method can be employed regardless ofa gray scale value included in image data, so that quality of movingimages can be improved. A sub-image may be obtained by direct gammaconversion of image data. In this case, a gamma value can be variouslycontrolled depending on the amount of movement of moving images or thelike. Furthermore, a sub-image whose gamma value is changed may beobtained by change in reference voltage of a digital-analog convertercircuit (DAC), without direct gamma conversion of image data. In thiscase, since the image data is not directly subjected to gammaconversion, an operation of a circuit which performs gamma conversioncan be stopped or the circuit itself can be omitted from a device; thus,power consumption and manufacturing cost of the device can be reduced.In the gamma correction method, since change in brightness L_(j) of eachsub-image with respect to gray scales depends on a gamma curve, eachsub-image itself can smoothly express the gray scale, and quality ofimages which are finally perceived by human eyes can be improved. Whenthe display device is an active matrix liquid crystal display device, aproblem of lack of writing voltage due to dynamic capacitance can beavoided; thus, quality of moving images can be significantly improved,in particular with respect to defects such as an afterimage and aphenomenon of a moving image in which traces are seen. Furthermore, acombination of 120 Hz driving and alternating-current driving of aliquid crystal display device is effective. That is, when drivingfrequency of the liquid crystal display device is 120 Hz and frequencyof alternating-current driving is an integer multiple of 120 Hz or aunit fraction of 120 Hz (e.g., 30 Hz, 60 Hz, 120 Hz, or 240 Hz),flickers which appear in alternating-current driving can be reduced to alevel that cannot be perceived by human eyes. Moreover, when the drivingmethod is applied to a liquid crystal display device in which responsetime of a liquid crystal element is approximately half a cycle of inputimage data, image quality can be improved.

For example, when n=2 and m=1, that is, the conversion ratio (n/m) is 2in the first step, a driving method as shown in the point where n=2 andm=1 in FIG. 14 is performed. At this time, the display frame rate isfour times a frame rate of image data to be input (quadruple-frame ratedriving). Specifically, for example, when the input frame rate is 60 Hz,the display frame rate is 240 Hz (240 Hz driving). Accordingly, fourimages are continuously displayed with respect to one piece of imagedata to be input. At this time, when an interpolation image in the firststep is an intermediate image obtained by motion compensation, themovement of moving images can be made to be smooth; thus, quality of themoving image can be significantly improved. When quadruple-frame ratedriving is performed, quality of moving images can be improved comparedwith the case where the frame rate is lower than that of quadruple-framerate driving, and power consumption and manufacturing cost can bereduced compared with the case where the frame rate is higher than thatof quadruple-frame rate driving. Further, in the procedure 1 of thesecond step, when a gamma correction method is selected among methodswhere brightness of an original image is divided between a plurality ofsub-images, an operation of a circuit which forms an intermediate imageby motion compensation can be stopped or the circuit itself can beomitted from a device; thus, power consumption and manufacturing cost ofthe device can be reduced. Moreover, a pseudo impulse display method canbe employed regardless of a gray scale value included in image data, sothat quality of moving images can be improved. A sub-image may beobtained by direct gamma conversion of image data. In this case, a gammavalue can be variously controlled depending on the amount of movement ofmoving images or the like. Furthermore, a sub-image whose gamma value ischanged may be obtained by change in reference voltage of adigital-analog converter circuit (DAC), without direct gamma conversionof image data. In this case, since the image data is not directlysubjected to gamma conversion, an operation of a circuit which performsgamma conversion can be stopped or the circuit itself can be omittedfrom a device; thus, power consumption and manufacturing cost of thedevice can be reduced. In the gamma correction method, since change inbrightness L_(j) of each sub-image with respect to gray scales dependson a gamma curve, each sub-image itself can smoothly express the grayscale, and quality of images which are finally perceived by human eyescan be improved. When the display device is an active matrix liquidcrystal display device, a problem of lack of writing voltage due todynamic capacitance can be avoided; thus, quality of moving images canbe significantly improved, in particular with respect to defects such asan afterimage and a phenomenon of a moving image in which traces areseen. Furthermore, a combination of 240 Hz driving andalternating-current driving of a liquid crystal display device iseffective. That is, when driving frequency of the liquid crystal displaydevice is 240 Hz and frequency of alternating-current driving is aninteger multiple of 240 Hz or a unit fraction of 240 Hz (e.g., 30 Hz, 60Hz, 120 Hz, or 240 Hz), flickers which appear in alternating-currentdriving can be reduced to a level that cannot be perceived by humaneyes. Moreover, when the driving method is applied to a liquid crystaldisplay device in which response time of a liquid crystal element isapproximately 1/4 of a cycle of input image data, image quality can beimproved.

For example, in the first step, when n=3 and m=1, that is, when theconversion ratio (n/m) is 3, a driving method as shown in the pointwhere n=3 and m=1 in FIG. 14 is performed. At this time, the displayframe rate is six times a frame rate of image data to be input (6-foldframe rate driving). Specifically, for example, when the input framerate is 60 Hz, the display frame rate is 360 Hz (360 Hz driving).Accordingly, six images are continuously displayed with respect to onepiece of image data to be input. At this time, when an interpolationimage in the first step is an intermediate image obtained by motioncompensation, the movement of moving images can be made to be smooth;thus, quality of the moving image can be significantly improved. When6-fold frame rate driving is performed, quality of moving images can beimproved compared with the case where the frame rate is lower than thatof 6-fold frame rate driving, and power consumption and manufacturingcost can be reduced compared with the case where the frame rate ishigher than that of 6-fold frame rate driving. Further, in the procedure1 of the second step, when a gamma correction method is selected amongmethods where brightness of an original image is divided between aplurality of sub-images, an operation of a circuit which forms anintermediate image by motion compensation can be stopped or the circuititself can be omitted from a device; thus, power consumption andmanufacturing cost of the device can be reduced. Moreover, a pseudoimpulse display method can be employed regardless of a gray scale valueincluded in image data, so that quality of moving images can beimproved. A sub-image may be obtained by direct gamma conversion ofimage data. In this case, a gamma value can be variously controlleddepending on the amount of movement of moving images or the like.Furthermore, a sub-image whose gamma value is changed may be obtained bychange in reference voltage of a digital-analog converter circuit (DAC),without direct gamma conversion of image data. In this case, since theimage data is not directly subjected to gamma conversion, an operationof a circuit which performs gamma conversion can be stopped or thecircuit itself can be omitted from a device; thus, power consumption andmanufacturing cost of the device can be reduced. In the gamma correctionmethod, since change in brightness L_(j) of each sub-image with respectto gray scales depends on a gamma curve, each sub-image itself cansmoothly express the gray scale, and quality of images which are finallyperceived by human eyes can be improved. Further, when the displaydevice is an active matrix liquid crystal display device, a problem oflack of writing voltage due to dynamic capacitance can be avoided; thus,quality of moving images can be significantly improved, in particularwith respect to defects such as an afterimage and a phenomenon of amoving image in which traces are seen. Furthermore, a combination of 360Hz driving and alternating-current driving of a liquid crystal displaydevice is effective. That is, when driving frequency of the liquidcrystal display device is 360 Hz and frequency of alternating-currentdriving is an integer multiple of 360 Hz or a unit fraction of 360 Hz(e.g., 30 Hz, 60 Hz, 120 Hz, or 180 Hz), flickers which appear inalternating-current driving can be reduced to a level that cannot beperceived by human eyes. Moreover, when the driving method is applied toa liquid crystal display device in which response time of a liquidcrystal element is approximately 1/6 of a cycle of input image data,image quality can be improved.

For example, in the first step, when n=3 and m=2, that is, when theconversion ratio (n/m) is 3/2, a driving method as shown in the pointwhere n=3 and m=2 in FIG. 14 is performed. At this time, the displayframe rate is three times a frame rate of image data to be input(triple-frame rate driving). Specifically, for example, when the inputframe rate is 60 Hz, the display frame rate is 180 Hz (180 Hz driving).Accordingly, three images are continuously displayed with respect to onepiece of image data to be input. At this time, when an interpolationimage in the first step is an intermediate image obtained by motioncompensation, the movement of moving images can be made to be smooth;thus, quality of the moving image can be significantly improved. Whentriple-frame rate driving is performed, quality of moving images can beimproved compared with the case where the frame rate is lower than thatof triple-frame rate driving, and power consumption and manufacturingcost can be reduced compared with the case where the frame rate ishigher than that of triple-frame rate driving. Further, in the procedure1 of the second step, when a gamma correction method is selected amongmethods where brightness of an original image is divided between aplurality of sub-images, an operation of a circuit which forms anintermediate image by motion compensation can be stopped or the circuititself can be omitted from a device; thus, power consumption andmanufacturing cost of the device can be reduced. Moreover, a pseudoimpulse display method can be employed regardless of a gray scale valueincluded in image data, so that quality of moving images can beimproved. A sub-image may be obtained by direct gamma conversion ofimage data. In this case, a gamma value can be variously controlleddepending on the amount of movement of moving images or the like.Furthermore, a sub-image whose gamma value is changed may be obtained bychange in reference voltage of a digital-analog converter circuit (DAC),without direct gamma conversion of image data. In this case, since theimage data is not directly subjected to gamma conversion, an operationof a circuit which performs gamma conversion can be stopped or thecircuit itself can be omitted from a device; thus, power consumption andmanufacturing cost of the device can be reduced. In the gamma correctionmethod, since change in brightness L_(j) of each sub-image with respectto gray scales depends on a gamma curve, each sub-image itself cansmoothly express the gray scale, and quality of images which are finallyperceived by human eyes can be improved. Further, when the displaydevice is an active matrix liquid crystal display device, a problem oflack of writing voltage due to dynamic capacitance can be avoided; thus,quality of moving images can be significantly improved, in particularwith respect to defects such as an afterimage and a phenomenon of amoving image in which traces are seen. Furthermore, a combination of 180Hz driving and alternating-current driving of a liquid crystal displaydevice is effective. That is, when driving frequency of the liquidcrystal display device is 180 Hz and frequency of alternating-currentdriving is an integer multiple of 180 Hz or a unit fraction of 180 Hz(e.g., 30 Hz, 60 Hz, 120 Hz, or 180 Hz), flickers which appear inalternating-current driving can be reduced to a level that cannot beperceived by human eyes. Moreover, when the driving method is applied toa liquid crystal display device in which response time of a liquidcrystal element is approximately 1/3 of a cycle of input image data,image quality can be improved.

For example, in the first step, when n=4 and m=1, that is, when theconversion ratio (n/m) is 4, a driving method as shown in the pointwhere n=4 and m=1 in FIG. 14 is performed. At this time, the displayframe rate is eight times a frame rate of image data to be input (8-foldframe rate driving). Specifically, for example, when the input framerate is 60 Hz, the display frame rate is 480 Hz (480 Hz driving).Accordingly, eight images are continuously displayed with respect to onepiece of image data to be input. At this time, when an interpolationimage in the first step is an intermediate image obtained by motioncompensation, the movement of moving images can be made to be smooth;thus, quality of the moving image can be significantly improved. When8-fold frame rate driving is performed, quality of moving images can beimproved compared with the case where the frame rate is lower than thatof 8-fold frame rate driving, and power consumption and manufacturingcost can be reduced compared with the case where the frame rate ishigher than that of 8-fold frame rate driving. Further, in the procedure1 of the second step, when a gamma correction method is selected amongmethods where brightness of an original image is divided between aplurality of sub-images, an operation of a circuit which forms anintermediate image by motion compensation can be stopped or the circuititself can be omitted from a device; thus, power consumption andmanufacturing cost of the device can be reduced. Moreover, a pseudoimpulse display method can be employed regardless of a gray scale valueincluded in image data, so that quality of moving images can beimproved. A sub-image may be obtained by direct gamma conversion ofimage data. In this case, a gamma value can be variously controlleddepending on the amount of movement of moving images or the like.Furthermore, a sub-image whose gamma value is changed may be obtained bychange in reference voltage of a digital-analog converter circuit (DAC),without direct gamma conversion of image data. In this case, since theimage data is not directly subjected to gamma conversion, an operationof a circuit which performs gamma conversion can be stopped or thecircuit itself can be omitted from a device; thus, power consumption andmanufacturing cost of the device can be reduced. In the gamma correctionmethod, since change in brightness L_(j) of each sub-image with respectto gray scales depends on a gamma curve, each sub-image itself cansmoothly express the gray scale, and quality of images which are finallyperceived by human eyes can be improved. Further, when the displaydevice is an active matrix liquid crystal display device, a problem oflack of writing voltage due to dynamic capacitance can be avoided; thus,quality of moving images can be significantly improved, in particularwith respect to defects such as an afterimage and a phenomenon of amoving image in which traces are seen. Furthermore, a combination of 480Hz driving and alternating-current driving of a liquid crystal displaydevice is effective. That is, when driving frequency of the liquidcrystal display device is 480 Hz and frequency of alternating-currentdriving is an integer multiple of 480 Hz or a unit fraction of 480 Hz(e.g., 30 Hz, 60 Hz, 120 Hz, or 240 Hz), flickers which appear inalternating-current driving can be reduced to a level that cannot beperceived by human eyes. Moreover, when the driving method is applied toa liquid crystal display device in which response time of a liquidcrystal element is approximately 1/8 of a cycle of input image data,image quality can be improved.

For example, in the first step, when n=4 and m=3, that is, when theconversion ratio (n/m) is 4/3, a driving method as shown in the pointwhere n=4 and m=3 in FIG. 14 is performed. At this time, the displayframe rate is 8/3 times a frame rate of image data to be input (8/3-foldframe rate driving). Specifically, for example, when the input framerate is 60 Hz, the display frame rate is 160 Hz (160 Hz driving).Accordingly, eight images are continuously displayed with respect tothree pieces of image data to be input. At this time, when aninterpolation image in the first step is an intermediate image obtainedby motion compensation, the movement of moving images can be made to besmooth; thus, quality of the moving image can be significantly improved.When 8/3-fold frame rate driving is performed, quality of moving imagescan be improved compared with the case where the frame rate is lowerthan that of 8/3-fold frame rate driving, and power consumption andmanufacturing cost can be reduced compared with the case where the framerate is higher than that of 8/3-fold frame rate driving. Further, in theprocedure 1 of the second step, when a gamma correction method isselected among methods where brightness of an original image is dividedbetween a plurality of sub-images, an operation of a circuit which formsan intermediate image by motion compensation can be stopped or thecircuit itself can be omitted from a device; thus, power consumption andmanufacturing cost of the device can be reduced. Moreover, a pseudoimpulse display method can be employed regardless of a gray scale valueincluded in image data, so that quality of moving images can beimproved. A sub-image may be obtained by direct gamma conversion ofimage data. In this case, a gamma value can be variously controlleddepending on the amount of movement of moving images or the like.Furthermore, a sub-image whose gamma value is changed may be obtained bychange in reference voltage of a digital-analog converter circuit (DAC),without direct gamma conversion of image data. In this case, since theimage data is not directly subjected to gamma conversion, an operationof a circuit which performs gamma conversion can be stopped or thecircuit itself can be omitted from a device; thus, power consumption andmanufacturing cost of the device can be reduced. In the gamma correctionmethod, since change in brightness L_(j) of each sub-image with respectto gray scales depends on a gamma curve, each sub-image itself cansmoothly express the gray scale, and quality of images which are finallyperceived by human eyes can be improved. Further, when the displaydevice is an active matrix liquid crystal display device, a problem oflack of writing voltage due to dynamic capacitance can be avoided; thus,quality of moving images can be significantly improved, in particularwith respect to defects such as an afterimage and a phenomenon of amoving image in which traces are seen. Furthermore, a combination of 160Hz driving and alternating-current driving of a liquid crystal displaydevice is effective. That is, when driving frequency of the liquidcrystal display device is 160 Hz and frequency of alternating-currentdriving is an integer multiple of 160 Hz or a unit fraction of 160 Hz(e.g., 40 Hz, 80 Hz, 160 Hz, or 320 Hz), flickers which appear inalternating-current driving can be reduced to a level that cannot beperceived by human eyes. Moreover, when the driving method is applied toa liquid crystal display device in which response time of a liquidcrystal element is approximately 3/8 of a cycle of input image data,image quality can be improved.

For example, in the first step, when n=5 and m=1, that is, when theconversion ratio (n/m) is 5, a driving method as shown in the pointwhere n=5 and m=1 in FIG. 14 is performed. At this time, the displayframe rate is ten times a frame rate of image data to be input (10-foldframe rate driving). Specifically, for example, when the input framerate is 60 Hz, the display frame rate is 600 Hz (600 Hz driving).Accordingly, ten images are continuously displayed with respect to onepiece of image data to be input. At this time, when an interpolationimage in the first step is an intermediate image obtained by motioncompensation, the movement of moving images can be made to be smooth;thus, quality of the moving image can be significantly improved. When10-fold frame rate driving is performed, quality of moving images can beimproved compared with the case where the frame rate is lower than thatof 10-fold frame rate driving, and power consumption and manufacturingcost can be reduced compared with the case where the frame rate ishigher than that of 10-fold frame rate driving. Further, in theprocedure 1 of the second step, when a gamma correction method isselected among methods where brightness of an original image is dividedbetween a plurality of sub-images, an operation of a circuit which formsan intermediate image by motion compensation can be stopped or thecircuit itself can be omitted from a device; thus, power consumption andmanufacturing cost of the device can be reduced. Moreover, a pseudoimpulse display method can be employed regardless of a gray scale valueincluded in image data, so that quality of moving images can beimproved. A sub-image may be obtained by direct gamma conversion ofimage data. In this case, a gamma value can be variously controlleddepending on the amount of movement of moving images or the like.Furthermore, a sub-image whose gamma value is changed may be obtained bychange in reference voltage of a digital-analog converter circuit (DAC),without direct gamma conversion of image data. In this case, since theimage data is not directly subjected to gamma conversion, an operationof a circuit which performs gamma conversion can be stopped or thecircuit itself can be omitted from a device; thus, power consumption andmanufacturing cost of the device can be reduced. In the gamma correctionmethod, since change in brightness L_(j) of each sub-image with respectto gray scales depends on a gamma curve, each sub-image itself cansmoothly express the gray scale, and quality of images which are finallyperceived by human eyes can be improved. Further, when the displaydevice is an active matrix liquid crystal display device, a problem oflack of writing voltage due to dynamic capacitance can be avoided; thus,quality of moving images can be significantly improved, in particularwith respect to defects such as an afterimage and a phenomenon of amoving image in which traces are seen. Furthermore, a combination of 600Hz driving and alternating-current driving of a liquid crystal displaydevice is effective. That is, when driving frequency of the liquidcrystal display device is 600 Hz and frequency of alternating-currentdriving is an integer multiple of 600 Hz or a unit fraction of 600 Hz(e.g., 30 Hz, 60 Hz, 100 Hz, or 120 Hz), flickers which appear inalternating-current driving can be reduced to a level that cannot beperceived by human eyes. Moreover, when the driving method is applied toa liquid crystal display device in which response time of a liquidcrystal element is approximately 1/10 of a cycle of input image data,image quality can be improved.

For example, in the first step, when n=5 and m=2, that is, when theconversion ratio (n/m) is 5/2, a driving method as shown in the pointwhere n=5 and m=2 in FIG. 14 is performed. At this time, the displayframe rate is five times a frame rate of image data to be input (5-foldframe rate driving). Specifically, for example, when the input framerate is 60 Hz, the display frame rate is 300 Hz (300 Hz driving).Accordingly, five images are continuously displayed with respect to onepiece of image data to be input. At this time, when an interpolationimage in the first step is an intermediate image obtained by motioncompensation, the movement of moving images can be made to be smooth;thus, quality of the moving image can be significantly improved. When5-fold frame rate driving is performed, quality of moving images can beimproved compared with the case where the frame rate is lower than thatof 5-fold frame rate driving, and power consumption and manufacturingcost can be reduced compared with the case where the frame rate ishigher than that of 5-fold frame rate driving. Further, in the procedure1 of the second step, when a gamma correction method is selected amongmethods where brightness of an original image is divided between aplurality of sub-images, an operation of a circuit which forms anintermediate image by motion compensation can be stopped or the circuititself can be omitted from a device; thus, power consumption andmanufacturing cost of the device can be reduced. Moreover, a pseudoimpulse display method can be employed regardless of a gray scale valueincluded in image data, so that quality of moving images can beimproved. A sub-image may be obtained by direct gamma conversion ofimage data. In this case, a gamma value can be variously controlleddepending on the amount of movement of moving images or the like.Furthermore, a sub-image whose gamma value is changed may be obtained bychange in reference voltage of a digital-analog converter circuit (DAC),without direct gamma conversion of image data. In this case, since theimage data is not directly subjected to gamma conversion, an operationof a circuit which performs gamma conversion can be stopped or thecircuit itself can be omitted from a device; thus, power consumption andmanufacturing cost of the device can be reduced. In the gamma correctionmethod, since change in brightness L_(j) of each sub-image with respectto gray scales depends on a gamma curve, each sub-image itself cansmoothly express the gray scale, and quality of images which are finallyperceived by human eyes can be improved. Further, when the displaydevice is an active matrix liquid crystal display device, a problem oflack of writing voltage due to dynamic capacitance can be avoided; thus,quality of moving images can be significantly improved, in particularwith respect to defects such as an afterimage and a phenomenon of amoving image in which traces are seen. Furthermore, a combination of 300Hz driving and alternating-current driving of a liquid crystal displaydevice is effective. That is, when driving frequency of the liquidcrystal display device is 300 Hz and frequency of alternating-currentdriving is an integer multiple of 300 Hz or a unit fraction of 300 Hz(e.g., 30 Hz, 50 Hz, 60 Hz, or 100 Hz), flickers which appear inalternating-current driving can be reduced to a level that cannot beperceived by human eyes. Moreover, when the driving method is applied toa liquid crystal display device in which response time of a liquidcrystal element is approximately 1/5 of a cycle of input image data,image quality can be improved.

As described above, when a gamma correction method is selected amongmethods where brightness of an original image is divided between aplurality of sub-images in the procedure 1 of the second step, thenumber of sub-images is decided to be 2 in the procedure 2 of the secondstep, and T₁=T₂=T/2 is decided in the procedure 3 of the second step,the frame rate can be twice that of frame rate conversion of theconversion ratio decided by values of n and m in the first step; thus,quality of moving images can be further improved. Further, quality ofmoving images can be improved compared with the case where a displayframe rate is lower than the display frame rate, and power consumptionand manufacturing cost can be reduced compared with the case where adisplay frame rate is higher than the display frame rate. Further, inthe procedure 1 of the second step, when a gamma correction method isselected among methods where brightness of an original image is dividedbetween a plurality of sub-images, an operation of a circuit which formsan intermediate image by motion compensation can be stopped or thecircuit itself can be omitted from a device; thus, power consumption andmanufacturing cost of the device can be reduced. Moreover, a pseudoimpulse display method can be employed regardless of a gray scale valueincluded in image data, so that quality of moving images can beimproved. A sub-image may be obtained by direct gamma conversion ofimage data. In this case, a gamma value can be variously controlleddepending on the amount of movement of moving images or the like.Furthermore, a sub-image whose gamma value is changed may be obtained bychange in reference voltage of a digital-analog converter circuit (DAC),without direct gamma conversion of image data. In this case, since theimage data is not directly subjected to gamma conversion, an operationof a circuit which performs gamma conversion can be stopped or thecircuit itself can be omitted from a device; thus, power consumption andmanufacturing cost of the device can be reduced. In the gamma correctionmethod, since change in brightness L_(j) of each sub-image with respectto gray scales depends on a gamma curve, each sub-image itself cansmoothly express the gray scale, and quality of images which are finallyperceived by human eyes can be improved. Further, when the displaydevice is an active matrix liquid crystal display device, a problem oflack of writing voltage due to dynamic capacitance can be avoided; thus,quality of moving images can be significantly improved, in particularwith respect to defects such as an afterimage and a phenomenon of amoving image in which traces are seen. Furthermore, when drivingfrequency of the liquid crystal display device is increased andfrequency of alternating-current driving is an integer multiple of thedriving frequency or a unit fraction of the driving frequency, flickerswhich appear in alternating-current driving can be reduced to a levelthat cannot be perceived by human eyes. Moreover, when the drivingmethod is applied to a liquid crystal display device in which responsetime of a liquid crystal element is approximately 1/(twice theconversion ratio) of a cycle of input image data, image quality can beimproved.

Note that although detailed description is omitted, it is apparent thatconversion ratios other than those described above have similaradvantages. For example, in the range that n is equal to or less than10, the combinations can be considered as follows other than thosedescribed above: n=5 and m=3, that is, conversion ratio (n/m) of 5/3(10/3-fold frame rate driving, 200 Hz); n=5 and m=4, that is, conversionratio (n/m) of 5/4 (5/2-fold frame rate driving, 150 Hz); n=6 and m=1,that is, conversion ratio (n/m) of 6 (12-fold frame rate driving, 720Hz); n=6 and m=5, that is, conversion ratio (n/m) of 6/5 (12/5-foldframe rate driving, 144 Hz); n=7 and m=1, that is, conversion ratio(n/m) of 7 (14-fold frame rate driving, 840 Hz); n=7 and m=2, that is,conversion ratio (n/m) of 7/2 (7-fold frame rate driving, 420 Hz); n=7and m=3, that is, conversion ratio (n/m) of 7/3 (14/3-fold frame ratedriving, 280 Hz); n=7 and m=4, that is, conversion ratio (n/m) of 7/4(7/2-fold frame rate driving, 210 Hz); n=7 and m=5, that is, conversionratio (n/m) of 7/5 (14/5-fold frame rate driving, 168 Hz); n=7 and m=6,that is, conversion ratio (n/m) of 7/6 (7/3-fold frame rate driving, 140Hz); n=8 and m=1, that is, conversion ratio (n/m) of 8 (16-fold framerate driving, 960 Hz); n=8 and m=3, that is, conversion ratio (n/m) of8/3 (16/3-fold frame rate driving, 320 Hz); n=8 and m=5, that is,conversion ratio (n/m) of 8/5 (16/5-fold frame rate driving, 192 Hz);n=8 and m=7, that is, conversion ratio (n/m) of 8/7 (16/7-fold framerate driving, 137 Hz); n=9 and m=1, that is, conversion ratio (n/m) of 9(18-fold frame rate driving, 1080 Hz); n=9 and m=2, that is, conversionratio (n/m) of 9/2 (9-fold frame rate driving, 540 Hz); n=9 and m=4,that is, conversion ratio (n/m) of 9/4 (9/2-fold frame rate driving, 270Hz); n=9 and m=5, that is, conversion ratio (n/m) of 9/5 (18/5-foldframe rate driving, 216 Hz); n=9 and m=7, that is, conversion ratio(n/m) of 9/7 (18/7-fold frame rate driving, 154 Hz); n=9 and m=8, thatis, conversion ratio (n/m) of 9/8 (9/4-fold frame rate driving, 135 Hz);n=10 and m=1, that is, conversion ratio (n/m) of 10 (20-fold frame ratedriving, 1200 Hz); n=10 and m=3, that is, conversion ratio (n/m) of 10/3(20/3-fold frame rate driving, 400 Hz); n=10 and m=7, that is,conversion ratio (n/m) of 10/7 (20/7-fold frame rate driving, 171 Hz);and n=10 and m=9, that is, conversion ratio (n/m) of 10/9 (20/9-foldframe rate driving, 133 Hz). Note that these frequencies are examples inthe case where the input frame rate is 60 Hz. In the case of otherfrequencies, the driving frequency is obtained by multiplication of thedoubled conversion ratio by the input frame rate.

Although specific numbers of n and m in the case where n is an integerlarger than 10 are not shown, it is apparent that the processes in thesecond step can be applied to a variety of combinations of n and m.

Although this embodiment mode describes the case where the number J ofsub-images is decided to be 2 in the procedure 2 and T₁=T₂=T/2 isdecided in the procedure 3, it is apparent that the invention is notlimited thereto.

For example, when T₁<T₂ is decided in the procedure 3 of the secondstep, the first sub-image can be made brighter and the second sub-imagecan be made darker. Further, when T₁>T₂ is decided in the procedure 3 ofthe second step, the first sub-image can be made darker and the secondsub-image can be made brighter. Accordingly, an original image can bewell perceived by human eyes, and at the same time, display can beperformed by pseudo impulse driving; thus, quality of moving images canbe improved. Note that when a gamma correction method is selected in theprocedure 1 among methods where brightness of an original image isdivided between a plurality of sub-images, as in the above-describeddriving method, a gamma value may be changed in changing brightness ofthe sub-image. That is, the gamma value may be determined depending ondisplay timing of the second sub-image. Accordingly, an operation of acircuit which changes brightness of the whole image can be stopped orthe circuit itself can be omitted from a device; thus, power consumptionand manufacturing cost of the device can be reduced.

In addition, in the procedure 2, it is apparent that the number J ofsub-images is not limited to 2, and values other than 2 may be employed.In this case, the display frame rate can be J times the frame rate offrame rate conversion of the conversion ratio determined by the valuesof n and m in the first step; thus, quality of moving images can befurther improved. Further, quality of moving images can be improvedcompared with the case where a display frame rate is lower than thedisplay frame rate, and power consumption and manufacturing cost can bereduced compared with the case where a display frame rate is higher thanthe display frame rate. Further, in the procedure 1 of the second step,when a gamma correction method is selected among methods wherebrightness of an original image is divided between a plurality ofsub-images, an operation of a circuit which forms an intermediate imageby motion compensation can be stopped or the circuit itself can beomitted from a device; thus, power consumption and manufacturing cost ofthe device can be reduced. Moreover, a pseudo impulse display method canbe employed regardless of a gray scale value included in image data, sothat quality of moving images can be improved. A sub-image may beobtained by direct gamma conversion of image data. In this case, a gammavalue can be variously controlled depending on the amount of movement ofmoving images or the like. Furthermore, a sub-image whose gamma value ischanged may be obtained by change in reference voltage of adigital-analog converter circuit (DAC), without direct gamma conversionof image data. In this case, since the image data is not directlysubjected to gamma conversion, an operation of a circuit which performsgamma conversion can be stopped or the circuit itself can be omittedfrom a device; thus, power consumption and manufacturing cost of thedevice can be reduced. In the gamma correction method, since change inbrightness L_(j) of each sub-image with respect to gray scales dependson a gamma curve, each sub-image itself can smoothly express the grayscale, and quality of images which are finally perceived by human eyescan be improved. Further, when the display device is an active matrixliquid crystal display device, a problem of lack of writing voltage dueto dynamic capacitance can be avoided; thus, quality of moving imagescan be significantly improved, in particular with respect to defectssuch as an afterimage and a phenomenon of a moving image in which tracesare seen. Furthermore, when driving frequency of the liquid crystaldisplay device is increased and frequency of alternating-current drivingis an integer multiple of the driving frequency or a unit fraction ofthe driving frequency, flickers which appear in alternating-currentdriving can be reduced to a level that cannot be perceived by humaneyes. Moreover, when the driving method is applied to a liquid crystaldisplay device in which response time of a liquid crystal element isapproximately 1/(J times of the conversion ratio) of a cycle of inputimage data, image quality can be improved.

For example, in the case where J=3, in particular, quality of movingimages can be improved compared with the case where the number ofsub-images is less than 3. Moreover, power consumption and manufacturingcost can be reduced compared with the case where the number ofsub-images is more than 3. Further, when the driving method is appliedto a liquid crystal display device in which response time of a liquidcrystal element is approximately 1/(three times of the conversion ratio)of a cycle of input image data, image quality can be improved.

For example, in the case where J=4, in particular, quality of movingimages can be improved compared with the case where the number ofsub-images is less than 4. Moreover, power consumption and manufacturingcost can be reduced compared with the case where the number ofsub-images is more than 4. Further, when the driving method is appliedto a liquid crystal display device in which response time of a liquidcrystal element is approximately 1/(four times of the conversion ratio)of a cycle of input image data, image quality can be improved.

In addition, for example, in the case where J=5, in particular, qualityof moving images can be improved compared with the case where the numberof sub-images is less than 5. Moreover, power consumption andmanufacturing cost can be reduced compared with the case where thenumber of sub-images is more than 5. Further, when the driving method isapplied to a liquid crystal display device in which response time of aliquid crystal element is approximately 1/(five times of the conversionratio) of a cycle of input image data, image quality can be improved.

Further, J other than those described above has similar advantages.

Next, another example of a driving method determined by the processes inthe second step is described in detail.

When a method in which an intermediate image obtained by motioncompensation is used as a sub-image is selected in the procedure 1 ofthe second step, the number J of sub-images is decided to be 2 in theprocedure 2, and T₁=T₂=T/2 is decided in the procedure 3, a drivingmethod determined by the processes in the second step is as follows.

In a driving method of a display device in which the i-th image data (iis a positive integer) and the (i+1)th image data are sequentiallyprepared in a certain period T, and the k-th image (k is a positiveinteger), the (k+1)th image, and the (k+2)th image are sequentiallydisplayed at an interval which is half a cycle of original image data,the k-th image is displayed in accordance with the i-th image data, the(k+1)th image is displayed in accordance with image data correspondingto movement obtained by multiplication of the amount of movement fromthe i-th image data to the (i+1)th image data by 1/2, and the (k+2)thimage is displayed in accordance with the (i+1)th image data. As theimage data which is sequentially prepared in the certain period T,original image data formed in the first step can be used. That is, allthe display patterns given in the explanation for the first step can becombined with the above-described driving method.

Note that in the above-described driving method, it is apparent that avariety of n and m used in the first step can be implemented incombination.

In the procedure 1 of the second step, an advantage in that the methodin which an intermediate image obtained by motion compensation is usedas a sub-image is selected is as follows. In the processes of the firststep, when an intermediate image obtained by motion compensation is usedas an interpolation image, a method for obtaining an intermediate imageused in the first step can also be used as it is in the second step.That is, a circuit used for obtaining an intermediate image by motioncompensation can be used not only in the first step but also in thesecond step; thus, the circuit can be effectively used, and processingefficiency can be improved. Moreover, movement of the image can be madesmoother, so that quality of moving images can be further improved.

Although this embodiment mode describes the case where the number J ofsub-images is decided to be 2 in the procedure 2 and T₁=T₂=T/2 isdecided in the procedure 3, it is apparent that the invention is notlimited thereto.

For example, when T₁<T₂ is decided in the procedure 3 of the secondstep, the first sub-image can be made brighter and the second sub-imagecan be made darker. Further, when T₁>T₂ is decided in the procedure 3 ofthe second step, the first sub-image can be made darker and the secondsub-image can be made brighter. Accordingly, an original image can bewell perceived by human eyes, and at the same time, display can beperformed by pseudo impulse driving; thus, quality of moving images canbe improved. Note that as in the above-described driving method, whenthe method in which an intermediate image obtained by motioncompensation is used as a sub-image is selected in the procedure 2,brightness of the sub-image is not needed to be changed. This is becausethe intermediate image is complete in itself as an image, and thus, theimage perceived by human eyes is not changed even when display timing ofthe second sub-image is changed. In this case, an operation of a circuitwhich changes brightness of the whole image can be stopped or thecircuit itself can be omitted from a device; thus, power consumption andmanufacturing cost of the device can be reduced.

In addition, in the procedure 2, it is apparent that the number J ofsub-images is not limited to 2, and values other than 2 may be employed.In this case, the display frame rate can be J times the frame rate offrame rate conversion of the conversion ratio determined by the valuesof n and m in the first step; thus, quality of moving images can befurther improved. Further, quality of moving images can be improvedcompared with the case where a display frame rate is lower than thedisplay frame rate, and power consumption and manufacturing cost can bereduced compared with the case where a display frame rate is higher thanthe display frame rate. In the case where the method where anintermediate image obtained by motion compensation is used as asub-image is selected in the procedure 1 of the second step, when anintermediate image obtained by motion compensation is used as aninterpolation image in the processes of the first step, a method forobtaining an intermediate image used in the first step can also be usedas it is in the second step. That is, a circuit used for obtaining anintermediate image by motion compensation can be used not only in thefirst step but also in the second step; thus, the circuit can beeffectively used, and processing efficiency can be improved. Moreover,movement of the image can be made smoother, so that quality of movingimages can be further improved. Further, when the display device is anactive matrix liquid crystal display device, a problem of lack ofwriting voltage due to dynamic capacitance can be avoided; thus, qualityof moving images can be significantly improved, in particular withrespect to defects such as an afterimage and a phenomenon of a movingimage in which traces are seen. Furthermore, when driving frequency ofthe liquid crystal display device is increased and frequency ofalternating-current driving is an integer multiple of the drivingfrequency or a unit fraction of the driving frequency, flickers whichappear in alternating-current driving can be reduced to a level thatcannot be perceived by human eyes. Moreover, when the driving method isapplied to a liquid crystal display device in which response time of aliquid crystal element is approximately 1/(J times of the conversionratio) of a cycle of input image data, image quality can be improved.

For example, in the case where J=3, in particular, quality of movingimages can be improved compared with the case where the number ofsub-images is less than 3. Moreover, power consumption and manufacturingcost can be reduced compared with the case where the number ofsub-images is more than 3. Further, when the driving method is appliedto a liquid crystal display device in which response time of a liquidcrystal element is approximately 1/(three times of the conversion ratio)of a cycle of input image data, image quality can be improved.

For example, in the case where J=4, in particular, quality of movingimages can be improved compared with the case where the number ofsub-images is less than 4. Moreover, power consumption and manufacturingcost can be reduced compared with the case where the number ofsub-images is more than 4. Further, when the driving method is appliedto a liquid crystal display device in which response time of a liquidcrystal element is approximately 1/(four times of the conversion ratio)of a cycle of input image data, image quality can be improved.

In addition, for example, in the case where J=5, in particular, qualityof moving images can be improved compared with the case where the numberof sub-images is less than 5. Moreover, power consumption andmanufacturing cost can be reduced compared with the case where thenumber of sub-images is more than 5. Further, when the driving method isapplied to a liquid crystal display device in which response time of aliquid crystal element is approximately 1/(five times of the conversionratio) of a cycle of input image data, image quality can be improved.

Further, J other than those described above has similar advantages.

Next, specific examples of a method for converting the frame rate whenthe input frame rate and the display frame rate are different aredescribed with reference to FIGS. 16A to 16C. In methods shown in FIGS.16A to 16C, circular regions in images are changed from frame to frame,and triangle regions in the images are hardly changed from frame toframe. Note that the images are only examples for explanation, and theimages to be displayed are not limited to these examples. The methodsshown in FIGS. 16A to 16C can be applied to a variety of images.

FIG. 16A shows the case where the display frame rate is twice as high asthe input frame rate (the conversion ratio is 2). When the conversionratio is 2, quality of moving images can be improved compared with thecase where the conversion ratio is less than 2. Further, when theconversion ratio is 2, power consumption and manufacturing cost can bereduced compared with the case where the conversion ratio is more than2. FIG. 16A schematically shows time change in images to be displayedwith time represented by the horizontal axis. Here, a focused image isreferred to as a p-th image p is a positive integer). An image displayedafter the focused image is referred to as a (p+1)th image, and an imagedisplayed before the focused image is referred to as a (p−1)th image,for example. Thus, how far an image to be displayed is apart from thefocused image is described for convenience. An image 1602 is the (p+1)thimage. An image 1603 is a (p+2)th image. An image 1604 is a (p+3)thimage. An image 1605 is a (p+4)th image. The period T_(in) represents acycle of input image data. Note that since FIG. 16A shows the case wherethe conversion ratio is 2, the period T_(in) is twice as long as aperiod after the p-th image is displayed until the (p+1)th image isdisplayed.

Here, the (p+1)th image 1602 may be an image which is made to be in anintermediate state between the p-th image 1601 and the (p+2)th image1603 by detecting the amount of change in the images from the p-th image1601 to the (p+2)th image 1603. FIG. 16A shows an image in anintermediate state by a region whose position is changed from frame toframe (the circular region) and a region whose position is hardlychanged from frame to frame (the triangle region). In other words, theposition of the circular region in the (p+1)th image 1602 is anintermediate position between the positions of the circular regions inthe p-th image 1601 and the (p+2)th image 1603. That is, as for the(p+1)th image 1602, image data is interpolated by motion compensation.When motion compensation is performed on a moving object on the image inthis manner to interpolate the image data, smooth display can beperformed.

Further, the (p+1)th image 1602 may be an image which is made to be inan intermediate state between the p-th image 1601 and the (p+2)th image1603 and may have luminance controlled by a certain rule. As the certainrule, for example, L>L_(c) may be satisfied when typical luminance ofthe p-th image 1601 is denoted by L and typical luminance of the (p+1)thimage 1602 is denoted by L_(c), as shown in FIG. 16A. Preferably,0.1L<L_(c)<0.8L is satisfied, and more preferably 0.2L<L_(c)<0.5L issatisfied. Alternatively, L<L_(c) may be satisfied, preferably0.1L_(c)<L<0.8L_(c) is satisfied, and more preferably0.2L_(c)<L<0.5L_(c) is satisfied. In this manner, display can be madepseudo impulse display, so that an afterimage perceived by human eyescan be suppressed.

Note that typical luminance of the images will be described later indetail with reference to FIGS. 17A to 17E.

When two different causes of motion blur (non-smoothness in movement ofimages and an afterimage perceived by human eyes) are removed at thesame time in this manner, motion blur can be considerably reduced.

Moreover, the (p+3)th image 1604 may also be formed from the (p+2)thimage 1603 and the (p+4)th image 1605 by using a similar method. Thatis, the (p+3)th image 1604 may be an image which is made to be in anintermediate state between the (p+2)th image 1603 and the (p+4)th image1605 by detecting the amount of change in the images from the (p+2)thimage 1603 to the (p+4)th image 1605 and may have luminance controlledby a certain rule.

FIG. 16B shows the case where the display frame rate is three times ashigh as the input frame rate (the conversion ratio is 3). FIG. 16Bschematically shows time change in images to be displayed with timerepresented by the horizontal axis. An image 1611 is the p-th image. Animage 1612 is the (p+1)th image. An image 1613 is a (p+2)th image. Animage 1614 is a (p+3)th image. An image 1615 is a (p+4)th image An image1616 is a (p+5)th image. An image 1617 is a (p+6)th image. The periodT_(in) represents a cycle of input image data. Note that since FIG. 16Bshows the case where the conversion ratio is 3, the period T_(in) isthree times as long as a period after the p-th image is displayed untilthe (p+1)th image is displayed.

Each of the (p+1)th image 1612 and the (p+2)th image 1613 may be animage which is made to be in an intermediate state between the p-thimage 1611 and the (p+3)th image 1614 by detecting the amount of changein the images from the p-th image 1611 to the (p+3)th image 1614. FIG.16B shows an image in an intermediate state by a region whose positionis changed from frame to frame (the circular region) and a region whoseposition is hardly changed from frame to frame (the triangle region).That is, the position of the circular region in each of the (p+1)thimage 1612 and the (p+2)th image 1613 is an intermediate positionbetween the positions of the circular regions in the p-th image 1611 andthe (p+3)th image 1614. Specifically, when the amount of movement of thecircular regions detected from the p-th image 1611 and the (p+3)th image1614 is denoted by X, the position of the circular region in the (p+1)thimage 1612 may be displaced by approximately (1/3)X from the position ofthe circular region in the p-th image 1611. Further, the position of thecircular region in the (p+2)th image 1613 may be displaced byapproximately (2/3)X from the position of the circular region in thep-th image 1611. That is, as for each of the (p+1)th image 1612 and the(p+2)th image 1613, image data is interpolated by motion compensation.When motion compensation is performed on a moving object on the image inthis manner to interpolate the image data, smooth display can beperformed.

Further, each of the (p+1)th image 1612 and the (p+2)th image 1613 maybe an image which is made to be in an intermediate state between thep-th image 1611 and the (p+3)th image 1614 and may have luminancecontrolled by a certain rule. As the certain rule, for example,L>L_(c1), L>L_(c2), or L_(c1)=L_(c2) may be satisfied when typicalluminance of the p-th image 1611 is denoted by L, typical luminance ofthe (p+1)th image 1612 is denoted by L_(c1), and typical luminance ofthe (p+2)th image 1613 is denoted by L_(c2), as shown in FIG. 16B.Preferably, 0.1L<L_(c1)=L_(c2)<0.8L is satisfied, and more preferably0.2L<L_(c)=L_(c2)<0.5L is satisfied. Alternatively, L<L_(c1), L<L_(c2),or L_(c1)=L_(c2) may be satisfied, preferably0.1L_(c1)=0.1L_(c2)<L=0.8L_(c1)=0.8L_(c2) is satisfied, and morepreferably 0.2L_(c1)=0.2L_(c2)<L=0.5L_(c1)=0.5L_(c2) is satisfied. Inthis manner, display can be made pseudo impulse display, so that anafterimage perceived by human eyes can be suppressed. Alternatively,images each of which luminance is changed may be made to appearalternately. In such a manner, a cycle of luminance change can beshortened, so that flickers can be reduced.

When two different causes of motion blur (non-smoothness in movement ofimages and an afterimage perceived by human eyes) are removed at thesame time in this manner, motion blur can be considerably reduced.

Moreover, each of the (p+4)th image 1615 and the (p+5)th image 1616 mayalso be formed from the (p+3)th image 1614 and the (p+6)th image 1617 byusing a similar method. That is, each of the (p+4)th image 1615 and the(p+5)th image 1616 may be an image which is made to be in anintermediate state between the (p+3)th image 1614 and the (p+6)th image1617 by detecting the amount of change in the images from the (p+3)thimage 1614 to the (p+6)th image 1617 and may have luminance controlledby a certain rule.

Note that when the method shown in FIG. 16B is used, the display framerate is so high that movement of the image can follow movement of humaneyes; thus, movement of the image can be displayed smoothly.Accordingly, motion blur can be considerably reduced.

FIG. 16C shows the case where the display frame rate is 1.5 times ashigh as the input frame rate (the conversion ratio is 1.5). FIG. 16Cschematically shows time change in images to be displayed with timerepresented by the horizontal axis. An image 1622 is the (p+1)th image.An image 1623 is the (p+2)th image. An image 1624 is the (p+3)th image.Note that although not necessarily displayed in practice, an image 1625,which is input image data, may be used to form the (p+1)th image 1622and the (p+2)th image 1623. The period T_(in) represents a cycle ofinput image data. Note that since FIG. 16C shows the case where theconversion ratio is 1.5, the period T_(in) is 1.5 times as long as aperiod after the p-th image is displayed until the (p+1)th image isdisplayed.

Here, each of the (p+1)th image 1622 and the (p+2)th image 1623 may bean image which is made to be in an intermediate state between the p-thimage 1621 and the (p+3)th image 1624 by detecting the amount of changein the images from the p-th image 1621 to the (p+3)th image 1624 via theimage 1625. FIG. 16C shows an image in an intermediate state by a regionwhose position is changed from frame to frame (the circular region) anda region whose position is hardly changed from frame to frame (thetriangle region). That is, the position of the circular region in eachof the (p+1)th image 1622 and the (p+2)th image 1623 is an intermediateposition between the positions of the circular regions in the p-th image1621 and the (p+3)th image 1624. That is, as for each of the (p+1)thimage 1622 and the (p+2)th image 1623, image data is interpolated bymotion compensation. When motion compensation is performed on a movingobject on the image in this manner to interpolate the image data, smoothdisplay can be performed.

Further, each of the (p+1)th image 1622 and the (p+2)th image 1623 maybe an image which is made to be in an intermediate state between thep-th image 1621 and the (p+3)th image 1624 and may have luminancecontrolled by a certain rule. As the certain rule, for example,L>L_(c1), L>L_(c2), or L_(c1)=L_(c2) is satisfied when typical luminanceof the p-th image 1621 is denoted by L, typical luminance of the (p+1)thimage 1622 is denoted by L_(c1), and typical luminance of the (p+2)thimage 1623 is denoted by L_(c2), as shown in FIG. 16C. Preferably,0.1L<L_(c1)=L_(c2)<0.8L is satisfied, and more preferably0.2L<L_(c1)=L_(c2)<0.5L is satisfied. Alternatively, L<L_(c1), L<L_(c2),or L_(c1)=L_(c2) may be satisfied, preferably0.1L_(c1)=0.1L_(c2)<L=0.8L_(c1)=0.8L_(c2) is satisfied, and morepreferably 0.2L_(c1)=0.2L_(c2)<L=0.5L_(c1)=0.5L_(c2) is satisfied. Inthis manner, display can be made pseudo impulse display, so that anafterimage perceived by human eyes can be suppressed. Alternatively,images each of which luminance is changed may be made to appearalternately. In this manner, a cycle of luminance change can beshortened, so that flickers can be reduced.

When two different causes of motion blur (non-smoothness in movement ofimages and an afterimage perceived by human eyes) are removed at thesame time in this manner, motion blur can be considerably reduced.

Note that when the method shown in FIG. 16C is used, the display framerate is so low that time for writing a signal to a display device can beincreased. Accordingly, clock frequency of the display device can bemade lower, so that power consumption can be reduced. Further,processing speed of motion compensation can be decreased, so that powerconsumption can be reduced.

Next, typical luminance of images is described with reference to FIGS.17A to 17E. FIGS. 17A to 17D each schematically show time change inimages to be displayed with time represented by the horizontal axis.FIG. 17E shows an example of a method for measuring luminance of animage in a certain region.

An example of a method for measuring luminance of an image includes amethod for individually measuring luminance of each pixel which formsthe image. With this method, luminance in every detail of the image canbe strictly measured.

Note that since a method for individually measuring luminance of eachpixel which forms the image needs much energy, another method may beused. Another method for measuring luminance of an image is a method formeasuring average luminance of a region in an image, which is focused.With this method, luminance of an image can be easily measured. In thisembodiment mode, luminance measured by a method for measuring averageluminance of a region in an image is referred to as typical luminance ofan image for convenience.

Then, which region in an image is focused in order to measure typicalluminance of the image is described below.

FIG. 17A shows an example of a measuring method in which luminance of aregion whose position is hardly changed with respect to change in animage (the triangle region) is typical luminance of the image. Theperiod T_(in) represents a cycle of input image data. An image 1701 isthe p-th image. An image 1702 is the (p+1)th image. An image 1703 is the(p+2)th image. A first region 1704 is a luminance measurement region inthe p-th image 1701. A second region 1705 is a luminance measurementregion in the (p+1)th image 1702. A third region 1706 is a luminancemeasurement region in the (p+2)th image 1703. Here, the first to thirdregions may be provided in almost the same spatial positions in adevice. That is, when typical luminance of the images is measured in thefirst to third regions, time change in typical luminance of the imagescan be calculated.

When the typical luminance of the images is measured, whether display ismade pseudo impulse display or not can be judged. For example, ifL_(c)<L is satisfied when luminance measured in the first region 1704 isdenoted by L and luminance measured in the second region 1705 is denotedby L_(c), it can be said that display is made pseudo impulse display. Atthat time, it can be said that quality of moving images is improved.

Note that when the amount of change in typical luminance of the imageswith respect to time change (relative luminance) in the luminancemeasurement regions is in the following range, image quality can beimproved. As for relative luminance, for example, relative luminancebetween the first region 1704 and the second region 1705 can be theratio of lower luminance to higher luminance; relative luminance betweenthe second region 1705 and the third region 1706 can be the ratio oflower luminance to higher luminance; and relative luminance between thefirst region 1704 and the third region 1706 can be the ratio of lowerluminance to higher luminance. That is, when the amount of change intypical luminance of the images with respect to time change (relativeluminance) is 0, relative luminance is 100%. When the relative luminanceis less than or equal to 80%, quality of moving images can be improved.In particular, when the relative luminance is less than or equal to 50%,quality of moving images can be significantly improved. Further, whenthe relative luminance is more than or equal to 10%, power consumptionand flickers can be reduced. In particular, when the relative luminanceis more than or equal to 20%, power consumption and flickers can besignificantly reduced. That is, when the relative luminance is more thanor equal to 10% and less than or equal to 80%, quality of moving imagescan be improved and power consumption and flickers can be reduced.Further, when the relative luminance is more than or equal to 20% andless than or equal to 50%, quality of moving images can be significantlyimproved and power consumption and flickers can be significantlyreduced.

FIG. 17B shows an example of a method in which luminance of regionswhich are divided into tiled shapes is measured and an average valuethereof is typical luminance of an image. The period T_(in) represents acycle of input image data. An image 1711 is the p-th image. An image1712 is the (p+1)th image. An image 1713 is the (p+2)th image. A firstregion 1714 is a luminance measurement region in the p-th image 1711. Asecond region 1715 is a luminance measurement region in the (p+1)thimage 1712. A third region 1716 is a luminance measurement region in the(p+2)th image 1713. Here, the first to third regions may be provided inalmost the same spatial positions in a device. That is, when typicalluminance of the images is measured in the first to third regions, timechange in typical luminance of the images can be measured.

When the typical luminance of the images is measured, whether display ismade pseudo impulse display or not can be judged. For example, ifL_(c)<L is satisfied when luminance measured in the first region 1714 isdenoted by L and luminance measured in the second region 1715 is denotedby L_(c), it can be said that display is made pseudo impulse display. Atthat time, it can be said that quality of moving images is improved.

Note that when the amount of change in typical luminance of the imageswith respect to time change (relative luminance) in the luminancemeasurement regions is in the following range, image quality can beimproved. As for relative luminance, for example, relative luminancebetween the first region 1714 and the second region 1715 can be theratio of lower luminance to higher luminance; relative luminance betweenthe second region 1715 and the third region 1716 can be the ratio oflower luminance to higher luminance; and relative luminance between thefirst region 1714 and the third region 1716 can be the ratio of lowerluminance to higher luminance. That is, when the amount of change intypical luminance of the images with respect to time change (relativeluminance) is 0, relative luminance is 100%. When the relative luminanceis less than or equal to 80%, quality of moving images can be improved.In particular, when the relative luminance is less than or equal to 50%,quality of moving images can be significantly improved. Further, whenthe relative luminance is more than or equal to 10%, power consumptionand flickers can be reduced. In particular, when the relative luminanceis more than or equal to 20%, power consumption and flickers can besignificantly reduced. That is, when the relative luminance is more thanor equal to 10% and less than or equal to 80%, quality of moving imagescan be improved and power consumption and flickers can be reduced.Further, when the relative luminance is more than or equal to 20% andless than or equal to 50%, quality of moving images can be significantlyimproved and power consumption and flickers can be significantlyreduced.

FIG. 17C shows an example of a method in which luminance of a centerregion in an image is measured and an average value thereof is typicalluminance of the image. The period T_(in) represents a cycle of inputimage data. An image 1721 is the p-th image. An image 1722 is the(p+1)th image. An image 1723 is the (p+2)th image. A first region 1724is a luminance measurement region in the p-th image 1721. A secondregion 1725 is a luminance measurement region in the (p+1)th image 1722.A third region 1726 is a luminance measurement region in the (p+2)thimage 1723.

When the typical luminance of the images is measured, whether display ismade pseudo impulse display or not can be judged. For example, ifL_(c)<L is satisfied when luminance measured in the first region 1724 isdenoted by L and luminance measured in the second region 1725 is denotedby L_(c), it can be said that display is made pseudo impulse display. Atthat time, it can be said that quality of moving images is improved.

Note that when the amount of change in typical luminance of the imageswith respect to time change (relative luminance) in the luminancemeasurement regions is in the following range, image quality can beimproved. As for relative luminance, for example, relative luminancebetween the first region 1724 and the second region 1725 can be theratio of lower luminance to higher luminance; relative luminance betweenthe second region 1725 and the third region 1726 can be the ratio oflower luminance to higher luminance; and relative luminance between thefirst region 1724 and the third region 1726 can be the ratio of lowerluminance to higher luminance. That is, when the amount of change intypical luminance of the images with respect to time change (relativeluminance) is 0, relative luminance is 100%. When the relative luminanceis less than or equal to 80%, quality of moving images can be improved.In particular, when the relative luminance is less than or equal to 50%,quality of moving images can be significantly improved. Further, whenthe relative luminance is more than or equal to 10%, power consumptionand flickers can be reduced. In particular, when the relative luminanceis more than or equal to 20%, power consumption and flickers can besignificantly reduced. That is, when the relative luminance is more thanor equal to 10% and less than or equal to 80%, quality of moving imagescan be improved and power consumption and flickers can be reduced.Further, when the relative luminance is more than or equal to 20% andless than or equal to 50%, quality of moving images can be significantlyimproved and power consumption and flickers can be significantlyreduced.

FIG. 17D shows an example of a method in which luminance of a pluralityof points sampled from the entire image is measured and an average valuethereof is typical luminance of the image. The period T_(in) representsa cycle of input image data. An image 1731 is the p-th image. An image1732 is the (p+1)th image. An image 1733 is the (p+2)th image. A firstregion 1734 is a luminance measurement region in the p-th image 1731. Asecond region 1735 is a luminance measurement region in the (p+1)thimage 1732. A third region 1736 is a luminance measurement region in the(p+2)th image 1733.

When the typical luminance of the images is measured, whether display ismade pseudo impulse display or not can be judged. For example, ifL_(c)<L is satisfied when luminance measured in the first region 1734 isdenoted by L and luminance measured in the second region 1735 is denotedby L_(c), it can be said that display is made pseudo impulse display. Atthat time, it can be said that quality of moving images is improved.

Note that when the amount of change in typical luminance of the imageswith respect to time change (relative luminance) in the luminancemeasurement regions is in the following range, image quality can beimproved. As for relative luminance, for example, relative luminancebetween the first region 1734 and the second region 1735 can be theratio of lower luminance to higher luminance; relative luminance betweenthe second region 1735 and the third region 1736 can be the ratio oflower luminance to higher luminance; and relative luminance between thefirst region 1734 and the third region 1736 can be the ratio of lowerluminance to higher luminance. That is, when the amount of change intypical luminance of the images with respect to time change (relativeluminance) is 0, relative luminance is 100%. When the relative luminanceis less than or equal to 80%, quality of moving images can be improved.In particular, when the relative luminance is less than or equal to 50%,quality of moving images can be significantly improved. Further, whenthe relative luminance is more than or equal to 10%, power consumptionand flickers can be reduced. In particular, when the relative luminanceis more than or equal to 20%, power consumption and flickers can besignificantly reduced. That is, when the relative luminance is more thanor equal to 10% and less than or equal to 80%, quality of moving imagescan be improved and power consumption and flickers can be reduced.Further, when the relative luminance is more than or equal to 20% andless than or equal to 50%, quality of moving images can be significantlyimproved and power consumption and flickers can be significantlyreduced.

FIG. 17E shows a measurement method in the luminance measurement regionsshown in FIGS. 17A to 17D. A region 1741 is a focused luminancemeasurement region. A point 1742 is a luminance measurement point in theluminance measurement region 1741. In a luminance measurement apparatushaving high time resolution, a measurement range thereof is small insome cases. Accordingly, in the case where the region 1741 is large,instead of measuring the whole region, a plurality of points in theregion 1741 may be measured uniformly by dots and an average valuethereof may be the luminance of the region 174, as shown in FIG. 17E.

Note that when the image is formed using a combination of three primarycolors of R, G, and B, luminance to be measured may be luminance of R, Gand B, luminance of R and G, luminance of G and B, luminance of B and R,or each luminance of R, G, and B.

Next, a method for producing an image in an intermediate state bydetecting movement of an image, which is included in input image data,and a method for controlling a driving method in accordance withmovement of an image, which is included in input image data, or the likeare described.

A method for producing an image in an intermediate state by detectingmovement of an image, which is included in input image data, isdescribed with reference to FIGS. 18A and 18B. FIG. 18A shows the casewhere the display frame rate is twice as high as the input frame rate(the conversion ratio is 2). FIG. 18A schematically shows a method fordetecting movement of an image with time represented by the horizontalaxis. The period T_(in) represents a cycle of input image data. An image1801 is the p-th image. An image 1802 is the (p+1)th image. An image1803 is the (p+2)th image. Further, as regions which are independent oftime, a first region 1804, a second region 1805, and a third region 1806are provided in images.

First, in the (p+2)th image 1803, the image is divided into a pluralityof tiled regions, and image data in the third region 1806 which is oneof the regions is focused.

Next, in the p-th image 1801, a region which uses the third region 1806as the center and is larger than the third region 1806 is focused. Here,the region which uses the third region 1806 as the center and is largerthan the third region 1806 corresponds to a data retrieval region. Inthe data retrieval region, a range in a horizontal direction (the Xdirection) is denoted by reference numeral 1807, and a range in aperpendicular direction (the Y direction) is denoted by referencenumeral 1808. Note that the range 1807 in the horizontal direction andthe range 1808 in the perpendicular direction may be ranges in whicheach of a range in a horizontal direction and a range in a perpendiculardirection of the third region 1806 is enlarged by approximately 15pixels.

Then, in the data retrieval region, a region having image data which ismost similar to the image data in the third region 1806 is retrieved. Asa retrieval method, a least-squares method or the like can be used. As aresult of retrieval, it is assumed that the first region 1804 is derivedas the region having the most similar image data.

Next, as an amount which shows positional difference between the derivedfirst region 1804 and the third region 1806, a vector 1809 is derived.Note that the vector 1809 is referred to as a motion vector.

Then, in the (p+1)th image 1802, the second region 1805 is formed by avector 1810 calculated from the motion vector 1809, the image data inthe third region 1806 in the (p+2)th image 1803, and image data in thefirst region 1804 in the p-th image 1801.

Here, the vector 1810 calculated from the motion vector 1809 is referredto as a displacement vector. The displacement vector 1810 has a functionof determining a position in which the second region 1805 is formed. Thesecond region 1805 is formed in a position which is apart from the thirdregion 1806 by the displacement vector 1810. Note that the amount of thedisplacement vector 1810 may be an amount which is obtained bymultiplication of the motion vector 1809 by a coefficient (1/2).

Image data in the second region 1805 in the (p+1)th image 1802 may bedetermined by the image data in the third region 1806 in the (p+2)thimage 1803 and the image data in the first region 1804 in the p-th image1801. For example, the image data in the second region 1805 in the(p+1)th image 1802 may be an average value between the image data in thethird region 1806 in the (p+2)th image 1803 and the image data in thefirst region 1804 in the p-th image 1801.

In such a manner, the second region 1805 in the (p+1)th image 1802,which corresponds to the third region 1806 in the (p+2)th image 1803,can be formed. Note that when the above-described treatment is alsoperformed on other regions in the (p+2)th image 1803, the (p+1)th image1802 which is made to be in an intermediate state between the (p+2)thimage 1803 and the p-th image 1801 can be formed.

FIG. 18B shows the case where the display frame rate is three times ashigh as the input frame rate (the conversion ratio is 3). FIG. 18Bschematically shows a method for detecting movement of an image withtime represented by the horizontal axis. The period T_(in) represents acycle of input image data. An image 1811 is the p-th image. An image1812 is the (p+1)th image. An image 1813 is the (p+2)th image. An image1814 is the (p+3)th image. Further, as regions which are independent oftime, a first region 1815, a second region 1816, a third region 1817,and a fourth region 1818 are provided in images.

First, in the (p+3)th image 1814, the image is divided into a pluralityof tiled regions, and image data in the fourth region 1818 which is oneof the regions is focused.

Next, in the p-th image 1811, a region which uses the fourth region 1818as the center and is larger than the fourth region 1818 is focused.Here, the region which uses the fourth region 1811 as the center and islarger than the fourth region 1818 corresponds to a data retrievalregion. In the data retrieval region, a range in a horizontal direction(the X direction) is denoted by reference numeral 1819, and a range in aperpendicular direction (the Y direction) is denoted by referencenumeral 1820. Note that the region 1819 in the horizontal direction andthe range 1820 in the perpendicular direction may be ranges in whicheach of a range in a horizontal direction and a range in a perpendiculardirection of the fourth region 1818 is enlarged by approximately 15pixels.

Then, in the data retrieval region, a region having image data which ismost similar to the image data in the fourth region 1818 is retrieved.As a retrieval method, a least-squares method or the like can be used.As a result of retrieval, it is assumed that the first region 1815 isderived as the region having the most similar image data.

Next, as an amount which shows positional difference between the derivedfirst region 1815 and the fourth region 1818, a vector 1821 is derived.Note that the vector 1821 is referred to as a motion vector.

Then, in each of the (p+1)th image 1812 and the (p+2)th image 1813, thesecond region 18016 and the third region 1817 are formed by a vector1822 and a vector 1823 calculated from the motion vector 1821, the imagedata in the fourth region 1818 in the (p+3)th image 1814, and image datain the first region 1815 in the p-th image 1811.

Here, the vector 1822 calculated from the motion vector 1821 is referredto as a first displacement vector. Moreover, the vector 1823 is referredto as a second displacement vector. The first displacement vector 1822has a function of determining a position in which the second region 1816is formed. The second region 1816 is formed in a position which is apartfrom the fourth region 1818 by the first displacement vector 1822. Notethat the first displacement vector 1822 may be an amount which isobtained by multiplication of the motion vector 1821 by a coefficient(1/3). Further, the second displacement vector 1823 has a function ofdetermining a position in which the third region 1817 is formed. Thethird region 1817 is formed in a position which is apart from the fourthregion 1818 by the second displacement vector 1823. Note that the seconddisplacement vector 1823 may be an amount which is obtained bymultiplication of the motion vector 1821 by a coefficient (2/3).

Image data in the second region 1816 in the (p+1)th image 1812 may bedetermined by the image data in the fourth region 1818 in the (p+3)thimage 1814 and the image data in the first region 1815 in the p-th image1811. For example, the image data in the second region 1816 in the(p+1)th image 1812 may be an average value between the image data in thefourth region 1818 in the (p+3)th image 1814 and the image data in thefirst region 1815 in the p-th image 1811.

Image data in the third region 1817 in the (p+2)th image 1813 may bedetermined by the image data in the fourth region 1818 in the (p+3)thimage 1814 and the image data in the first region 1815 in the p-th image1811. For example, the image data in the third region 1817 in the(p+2)th image 1813 may be an average value between the image data in thefourth region 1818 in the (p+3)th image 1814 and the image data in thefirst region 1815 in the p-th image 1811.

In such a manner, the second region 1816 in the (p+1)th image 1812 andthe third region 1817 in the (p+2)th image 1813, which correspond to thefourth region 1818 in the (p+3)th image 1814, can be formed. Note thatwhen the above-described treatment is also performed on other regions inthe (p+3)th image 1814, the (p+1)th image 1812 and the (p+2)th image1813 which are made to be in an intermediate state between the (p+3)thimage 1814 and the p-th image 1811 can be formed.

Next, an example of a circuit which produces an image in an intermediatestate by detecting movement of an image, which is included in inputimage data, is described with reference to FIGS. 19A to 19D. FIG. 19Ashows a connection relation between a peripheral driver circuitincluding a source driver and a gate driver for displaying an image on adisplay region, and a control circuit for controlling the peripheraldriver circuit. FIG. 19B shows an example of a specific circuitstructure of the control circuit. FIG. 19C shows an example of aspecific circuit structure of an image processing circuit included inthe control circuit. FIG. 19D shows another example of a specificcircuit structure of the image processing circuit included in thecontrol circuit.

As shown in FIG. 19A, a device in this embodiment mode may include acontrol circuit 1911, a source driver 1912, a gate driver 1913, and adisplay region 1914.

Note that the control circuit 1911, the source driver 1912, and the gatedriver 1913 may be formed over the same substrate as the display region1914.

Note that part of the control circuit 1911, the source driver 1912, andthe gate driver 1913 may be formed over the same substrate as thedisplay region 1914, and other circuits may be formed over a differentsubstrate from that of the display region 1914. For example, the sourcedriver 1912 and the gate driver 1913 may be formed over the samesubstrate as the display region 1914, and the control circuit 1911 maybe formed over a different substrate as an external IC. Alternatively,the gate driver 1913 may be formed over the same substrate as thedisplay region 1914, and other circuits may be formed over a differentsubstrate as an external IC. Similarly, part of the source driver 1912,the gate driver 1913, and the control circuit 1911 may be formed overthe same substrate as the display region 1914, and other circuits may beformed over a different substrate as an external IC.

The control circuit 1911 may have a structure to which an external imagesignal 1900, a horizontal synchronization signal 1901, and a verticalsynchronization signal 1902 are input and an image signal 1903, a sourcestart pulse 1904, a source clock 1905, a gate start pulse 1906, and agate clock 1907 are output.

The source driver 1912 may have a structure in which the image signal1903, the source start pulse 1904, and the source clock 1905 are inputand voltage or current in accordance with the image signal 1903 isoutput to the display region 1914.

The gate driver 1913 may have a structure in which the gate start pulse1906 and the gate clock 1907 are input and a signal which specifiestiming for writing a signal output from the source driver 1912 to thedisplay region 1914 is output.

When frequency of the external image signal 1900 is different fromfrequency of the image signal 1903, a signal for controlling timing fordriving the source driver 1912 and the gate driver 1913 is alsodifferent from frequency of the horizontal synchronization signal 1901and the vertical synchronization signal 1902 which are input.Accordingly, in addition to processing of the image signal 1903, it isnecessary to process the signal for controlling timing for driving thesource driver 1912 and the gate driver 1913. The control circuit 1911may have a function of processing the signal for controlling timing fordriving the source driver 1912 and the gate driver 1913. For example,when the frequency of the image signal 1903 is twice as high as thefrequency of the external image signal 1900, the control circuit 1911generates the image signal 1903 having the doubled frequency byinterpolating an image signal included in the external image signal1900, and controls the signal for controlling timing so that the signalalso has the doubled frequency.

Further, as shown in FIG. 19B, the control circuit 1911 may include animage processing circuit 1915 and a timing generation circuit 1916.

The image processing circuit 1915 may have a structure in which theexternal image signal 1900 and a frequency control signal 1908 are inputand the image signal 1903 is output.

The timing generation circuit 1916 may have a structure in which thehorizontal synchronization signal 1901 and the vertical synchronizationsignal 1902 are input, and the source start pulse 1904, the source clock1905, the gate start pulse 1906, the gate clock 1907, and the frequencycontrol signal 1908 are output. Note that the timing generation circuit1916 may include a memory, a register, or the like for holding data forspecifying the state of the frequency control signal 1908.Alternatively, the timing generation circuit 1916 may have a structurein which a signal for specifying the state of the frequency controlsignal 1908 is input.

As shown in FIG. 19C, the image processing circuit 1915 may include amotion detection circuit 1920, a first memory 1921, a second memory1922, a third memory 1923, a luminance control circuit 1924, and ahigh-speed processing circuit 1925.

The motion detection circuit 1920 may have a structure in which aplurality of pieces of image data are input, movement of an image isdetected, and image data which is in an intermediate state of theplurality of pieces of image data is output.

The first memory 1921 may have a structure in which the external imagesignal 1900 is input, the external image signal 1900 is held for acertain period, and the external image signal 1900 is output to themotion detection circuit 1920 and the second memory 1922.

The second memory 1922 may have a structure in which image data outputfrom the first memory 1921 is input, the image data is held for acertain period, and the image data is output to the motion detectioncircuit 1920 and the high-speed processing circuit 1925.

The third memory 1923 may have a structure in which image data outputfrom the motion detection circuit 1920 is input, the image data is heldfor a certain period, and the image data is output to the luminancecontrol circuit 1924.

The high-speed processing circuit 1925 may have a structure in whichimage data output from the second memory 1922, image data output fromthe luminance control circuit 1924, and the frequency control signal1908 are input and the image data is output as the image signal 1903.

When the frequency of the external image signal 1900 is different fromthe frequency of the image signal 1903, the image signal 1903 may begenerated by interpolating the image signal included in the externalimage signal 1900 by the image processing circuit 1915. The inputexternal image signal 1900 is once held in the first memory 1921. Atthat time, image data which is input in the previous frame is held inthe second memory 1922. The motion detection circuit 1920 may read theimage data held in the first memory 1921 and the second memory 1922 asappropriate to detect a motion vector by difference between the bothpieces of image data and to generate image data in an intermediatestate. The generated image data in an intermediate state is held in thethird memory 1923.

When the motion detection circuit 1920 generates the image data in anintermediate state, the high-speed processing circuit 1925 outputs theimage data held in the second memory 1922 as the image signal 1903.After that, the image data held in the third memory 1923 is output asthe image signal 1903 through the luminance control circuit 1924. Atthis time, frequency updated by the second memory 1922 and the thirdmemory 1923 is the same as the external image signal 1900; however, thefrequency of the image signal 1903 which is output through thehigh-speed processing circuit 1925 may be different from the frequencyof the external image signal 1900. Specifically, for example, thefrequency of the image signal 1903 is 1.5 times, twice, or three timesas high as the frequency of the external image signal 1900. However, thepresent invention is not limited to this, and a variety of frequenciescan be used. Note that the frequency of the image signal 1903 may bespecified by the frequency control signal 1908.

The structure of the image processing circuit 1915 shown in FIG. 19D isobtained by adding a fourth memory 1926 to the structure of the imageprocessing circuit 1915 shown in FIG. 19C. When image data output fromthe fourth memory 1926 is output to the motion detection circuit 1920 inaddition to the image data output from the first memory 1921 and theimage data output from the second memory 1922 in this manner, movementof an image can be detected adequately.

Note that when image data to be input has already included a motionvector for data compression or the like, for example, the image data tobe input is image data which is based on an MPEG (moving picture expertgroup) standard, an image in an intermediate state may be generated asan interpolated image by using this image data. At this time, a portionwhich generates a motion vector included in the motion detection circuit1920 is not necessary. Further, since encoding and decoding processingof the image signal 1903 is simplified, power consumption can bereduced.

Note that although this embodiment mode is described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in each drawing in this embodiment modewith part of another embodiment mode.

Note that this embodiment mode shows examples of embodying, slightlytransforming, partially modifying, improving, describing in detail, orapplying the contents (or part of the contents) described in anotherembodiment mode, an example of related part thereof, or the like.Therefore, the contents described in another embodiment mode can befreely applied to, combined with, or replaced with this embodiment mode.

Embodiment Mode 2

In this embodiment mode, application examples of a method where imagequality of a display device is improved, mainly change in driving methoddepending on circumstances are described. The circumstances here includecontents of image data, environment inside and outside the device (e.g.,temperature, humidity, barometric pressure, light, sound, electricfield, the amount of radiation, altitude, acceleration, or movementspeed), user settings, and software version.

A method in which movement of an image included in input image data isdetected to form an intermediate image is described with reference toFIGS. 20A to 20E. FIGS. 20A to 20E schematically illustrate imagesdisplayed when the display frame rate is twice as high as the inputframe rate. FIG. 20A schematically illustrates a method of detectingmotion of the image with time represented by the horizontal axis. Theperiod T_(in) represents a cycle of input image data. An image 2001represents the p-th image. An image 2002 represents the (p+1)th image.An image 2003 represents the (p+2)th image. In the images, a firstregion 2004, a second region 2005, a third region 2006, a fourth region2007, a fifth region 2008, and a sixth region 2009 are provided asregions which do not depend on time.

The method of obtaining the second region 2005 in the (p+1)th image 2002from the third region 2006 of the (p+2)th image 2003 and the firstregion 2004 of the p-th image 2001 may be the above-described method. Inother words, the p-th image 2001, the (p+1)th image 2002, the (p+2)thimage 2003, the first region 2004, the second region 2005, the thirdregion 2006, a movement vector 2010, and a displacement vector 2011 inFIG. 20A may correspond to the p-th image 1801, the (p+1)th image 1802,the (p−2)th image 1803, the first region 1804, the second region 1805,the third region 1806, the movement vector 1809, and the displacementvector 1810 in FIG. 18A, respectively.

Image data included in the fourth region 2007 and the sixth region 2009are hardly moved in the p-th image 2001 and the (p+2)th image 2003. Inthis case, image data produced in the fifth region 2008 may be anaverage value of the image data of the fourth region 2007 in the p-thimage 2001 and the image data of the sixth region 2009 in the (p+2)thimage 2003; however, as shown in FIG. 20A, the image data in the fifthregion 2008 may be a black image having low luminance. That is, anintermediate image may be produced for the region in which the image ismoved so much, and a black image may be produced for the region in whichthe image is not moved so much. Accordingly, typical luminance of the(p+1)th image 2002 becomes small, and display can be pseudo impulse typedisplay. In such a manner, the image is interpolated by motioncompensation, and then, display may be pseudo impulse type display byprovision of difference in typical luminances between images to bedisplayed. Thus, motion can be smoothed and afterimages can be reduced;thus, motion blur can be drastically reduced.

When whether an intermediate image or a black image is produced isdetermined, the movement vector 2010 may have a threshold value. Thethreshold value of the movement vector 2010 is preferably three times ashigh as that of one pixel, more preferably twice as high as that of onepixel.

FIG. 20B schematically illustrates shift of an image when anintermediate image is produced by the method shown in FIG. 20A. Theperiod T_(in) represents a cycle of input image data. An image 2021represents the p-th image. An image 2022 represents the (p+1)th image.An image 2023 represents the (p+2)th image. Each of arrows 2024, 2025,and 2026 represents scanning of an image by a scan signal.

FIG. 20B illustrates an image including both of a region whose positionis changed from frame to frame (the circular region) and a region whoseposition is hardly changed from frame to frame (the triangle region) asan example. At this time, in a driving method shown in FIG. 20B, the(p+1)th image 2022 is produced in different ways in a region where themovement vector is large and in a region where the movement vector issmall. The movement vector is detected based on the movement of theimage. Specifically, the intermediate image is employed in the regionwhere the movement vector is large (here, the circular region), and theblack image is employed in the region where the movement vector is small(here, the triangle region). In such a manner, typical luminance of the(p+1)th image 2022 is low so that display can be pseudo impulse typedisplay. In this manner, the image is interpolated by motioncompensation, and then, display may be pseudo impulse type display byprovision of difference in typical luminances between images to bedisplayed. Accordingly, motion can be smoothed and afterimages can bereduced; thus, motion blur can be drastically reduced.

Note that in FIG. 20B, when the image data of the (p+1)th image 2022 iswritten to a display region, instead of writing black data positively,the data may be written only to the region in which the image is movedlargely, while the data of the p-th image 2021 may be held in the regionin which the image is not moved so much, without writing the data of the(p+1)th image 2022 thereto. In this case, when the data of the p-thimage 2021 is written, a scan signal is scanned entirely as indicated bythe arrow 2024. Next, when the data of the (p+1)th image 2022 iswritten, the scan signal is scanned only in the region in which theimage is moved largely as indicated by the arrow 2025. When the data ofthe (p+2)th image 2023 is written, the scan signal is scanned entirelyas indicated by the arrow 2026. Accordingly, it is unnecessary to writedata in a region in which motion is small and the intermediate image isnot needed to be displayed, which leads to reduction in powerconsumption. Further, a noise is not mixed when the (p+1)th image 2022is produced, which leads to improvement in image quality.

FIGS. 20C to 20E illustrate images including both of a region whoseposition is changed from frame to frame (the circular region) and aregion whose position is hardly changed from frame to frame (thetriangle region) as an example. In FIGS. 20C to 20E, the amount ofmovement is different in the regions in which the images are moved fromframe to frame.

In FIG. 20C, the period T_(in) represents a cycle of input image data.An image 2031 represents the p-th image. An image 2032 represents the(p+1)th image. An image 2033 represents the (p+2)th image. The (p+1)thimage 2032 may be an image which is made to be in an intermediate statebetween the p-th image 2031 and (the p+2)th image 2033 by detection ofthe amount of change (a movement vector) of the image from the p-thimage 2031 to the (p+2)th image 2033. Further, the (p+1)th image 2032may be in an intermediate state between the p-th image 2031 and the(p−2)th image 2033 and may have luminance controlled by a certain rule.Specifically, the certain rule may be determined based on the amplitudeof the movement vector.

In FIG. 20D, the period T_(in) represents a cycle of input image data.An image 2041 represents the p-th image. An image 2042 represents the(p+1)th image. An image 2043 represents the (p+2)th image. The (p+1)thimage 2042 may be an image which is made to be an intermediate statebetween the p-th image 2041 and the (p+2)th image 2043 by detection ofthe amount of change (a movement vector) of the image from the p-thimage 2041 to the (p+2)th image 2043. Further, the (p+1)th image 2042may be an image which is made to be in an intermediate state between thep-th image 2041 and the (p+2)th image 2043 and may have luminancecontrolled by a certain rule. Specifically, the certain rule may bedetermined based on the amplitude of the movement vector.

In FIG. 20E, the period T_(in) represents a cycle of input image data.An image 2051 represents the p-th image. An image 2052 represents the(p+1)th image. An image 2053 represents the (p+2)th image. The (p+1)thimage 2052 may be an image which is made to be in an intermediate statebetween the p-th image 2051 and the (p+2)th image 2053 by detection ofthe amount of change (a movement vector) of the image from the p-thimage 2051 to the (p+2)th image 2053. Further, the (p+1)th image 2052may be an image which is made to be in an intermediate state between thep-th image 2051 and the (p+2)th image 2053 and may have luminancecontrolled by a certain rule. Specifically, the certain rule may bedetermined based on the amplitude of the movement vector.

In FIGS. 20C to 20E, the amount of movement of the region whose positionis changed from frame to frame (the circular region) is the largest inthe driving method shown in FIG. 20D, and is followed by the drivingmethod shown in FIG. 20C and the driving method shown in FIG. 20E. Atthis time, luminance of the interpolation image may be determined by themagnitude of the detected movement vector.

That is, when the luminance of the (p+1)th image 2032 in FIG. 20C isdenoted by L_(c0), the luminance of the (p+1)th image 2042 in FIG. 20Dis denoted by L_(c1), and the luminance of the (p+1)th image 2052 inFIG. 20E is denoted by L_(c2), a relation among L_(c0), L_(c1) andL_(c2) may satisfy L_(c1)<L_(c0)<L_(c2). In other words, as the amountof movement of the region whose position is changed from frame to frameis larger, the luminance of the interpolation image may be made smaller.Accordingly, when the amount of movement of the image is large and thusmotion blur occurs strongly, display can be made closer to pseudoimpulse type display. In such a manner, the image is interpolated bymotion interpolation, and then, display may be pseudo impulse typedisplay by provision of difference in typical luminances between imagesto be displayed. Accordingly, motion can be smoothed and afterimages canbe suppressed; thus, motion blur can be drastically reduced.

Note that when movement of an image is small and motion blur does notoccur so much, display can be close to hold type display; thus, flickersand power consumption can be reduced.

Note that the control amount of luminance of an image may be determinedby not only the amplitude of a detected movement vector but also settingby a user or external environment (e.g., surrounding brightness ortemperature), or a combination of the above.

Next, a method of relating movement of an image to a display frame rateis described with reference to FIGS. 21A to 21C. FIGS. 21A to 21Cschematically illustrate temporal change of an image to be displayedwith time represented by the horizontal axis. FIGS. 21A to 21C eachillustrate intermediate images by using a region whose position ischanged from frame to frame (a circular region) and a region whoseposition is hardly changed from frame to frame (a triangle region). FIG.21A shows the case where the display frame rate is twice as high as theinput frame rate.

FIG. 21B shows the case where the display frame rate is three times ashigh as the input frame rate. FIG. 21C shows the case where the displayframe rate is 1.5 times as high as the input frame rate.

In FIG. 21A, the period T_(in) represents a cycle of input image data.An image 2101 represents the p-th image. An image 2102 represents the(p+1)th image. An image 2103 represents the (p+2)th image. An image 2104represents the (p+3)th image. An image 2105 represents the (p+4)thimage.

In FIG. 21A, the (p+1)th image 2102 may be an image which is made to bein an intermediate state between the p-th image 2101 and the (p+2)thimage 2103 by detection of the amount of change of the image from thep-th image 2101 to the (p+2)th image 2103. Similarly, the (p+3)th image2104 may be an image which is made to be in an intermediate statebetween the (p+2)th image 2103 and the (p+4)th image 2105 by detectionof the amount of change of the image from the (p+2)th image 2103 to the(p+4)th image 2105.

In FIG. 21B, the period T_(in) represents a cycle of input image data.An image 2111 represents the p-th image. An image 2112 represents the(p+1)th image. An image 2113 represents the (p+2)th image. An image 2114represents the (p+3)th image. An image 2115 represents the (p+4)thimage. An image 2116 represents the (p+5)th image. An image 2117represents the (p+6)th image.

In FIG. 21B, each of the (p+1)th image 2112 and the (p+2)th image 2113may be an image which is made to be in an intermediate state between thep-th image 2111 and the (p+3)th image 2114 by detection of the amount ofchange of the image from the p-th image 2111 to the (p+3)th image 2114.Similarly, each of the (p+4)th image 2115 and the (p+5)th image 2116 maybe an image which is made to be in an intermediate state between the(p+3)th image 2114 and the (p+6)th image 2117 by detection of the amountof change of the image from the (p+3)th image 2114 to the (p+6)th image2117.

In FIG. 21C, the period T_(in) represents a cycle of input image data.An image 2121 represents the p-th image. An image 2122 represents the(p+1)th image. An image 2123 represents the (p+2)th image. An image 2124represents the (p+3)th image. Note that although not necessarilydisplayed in practice, an image 2125 which is input image data may beused to form the (p+1)th image 2122 and the (p+2)th image 2123.

In FIG. 21C, each of the (p+1)th image 2122 and the (p+2)th image 2123may be an image which is made to be in an intermediate state between thep-th image 2121 and the (p+3)th image 2124 by detection of the amount ofchange of the image from the p-th image 2121 to the (p+3)th image 2124via the (p+4)th image 2125.

In FIGS. 21A to 21C, the amount of movement of a region whose positionis moved from frame to frame in the basic image varies. In other words,the amount of movement of the image shown in FIG. 21B (the display framerate is three times as high as the input frame rate) is the largest, andis followed by that of the image shown in FIG. 21A (the display framerate is twice as high as the input frame rate) and that of the imageshown in FIG. 21C (the display frame rate is 1.5 times as high as theinput frame rate). In such a manner, in accordance with the amount ofmovement of the image, the frequency of the display frame rate may bechanged for display. Accordingly, driving frequency appropriate for theamount of movement of the image can be selected; thus, motion blur canbe effectively reduced by improvement in smoothness of moving images,and increase in heat generation due to increase in power consumption andprocessing amount can be reduced. Further, flickers can be reduced whenmovement of an image is small.

Note that the display frame rate may be determined by not only theamplitude of a detected movement vector but also setting by a user orexternal environment (e.g., surrounding brightness or temperature), or acombination of the above.

In addition, each of the interpolation images shown in FIGS. 21A to 21Cmay be an image which is made to be in an intermediate state between aplurality of basic images and may have luminance controlled by a certainrule. Specifically, the certain rule may be determined by the amplitudeof the movement vector, setting by a user, or external environment(e.g., surrounding brightness or temperature), or a combination of theabove. In such a manner, the image is interpolated by motioncompensation, and then, display may be pseudo impulse type display byprovision of difference in typical luminances between images to bedisplayed. Accordingly, motion can be smoothed and afterimages can bereduced; thus, motion blur can be drastically reduced.

Next, a method of relating movement of an image to a driving method isdescribed with reference to FIGS. 22A to 22E. FIGS. 22A to 22Eschematically illustrate temporal change of an image to be displayedwith time represented by the horizontal axis. FIGS. 22A to 22E eachillustrate an intermediate image by using a region whose position ischanged from frame to frame (a circular region) and a region whoseposition is hardly changed from frame to frame (a triangle region).

In FIG. 22A, the period T_(in) represents a cycle of input image data.An image 2201 represents the p-th image. An image 2202 represents the(p+1)th image. An image 2203 represents the (p+2)th image. Note that the(p+1)th image 2202 may be an image which is made to be in anintermediate state between the p-th image 2201 and the (p+2)th image2203 by detection of the amount of change (a movement vector) of theimage from the p-th image 2201 to the (p+2)th image 2203. Further, the(p+1)th image 2202 may be in an intermediate state between the p-thimage 2201 and the (p+2)th image 2203 and may have luminance controlledby a certain rule. Specifically, the certain rule may be determined bythe amplitude of the movement vector, setting by a user, or externalenvironment (e.g., surrounding brightness or temperature), or acombination of the above.

FIG. 22A illustrates the driving method in which the amount of movementof an image is detected from image data to be input and an intermediateimage between the images is used as an interpolation image, andluminance of the interpolation image is changed. In this embodimentmode, the driving method illustrated in FIG. 22A is referred to asluminance control double-frame rate driving.

In FIG. 22B, the period T_(in) represents a cycle of input image data.An image 2211 represents the p-th image. An image 2212 represents the(p+1)th image. An image 2213 represents the (p+2)th image. Note that the(p+1)th image 2212 may be an image which is made to be in anintermediate state between the p-th image 2211 and the (p+2)th image2213 by detection of the amount of change (a movement vector) of theimage from the p-th image 2211 to the (p+2)th image 2213. Further, the(p+1)th image 2212 may be in an intermediate state between the p-thimage 2211 and the (p+2)th image 2213 and may have a display frame ratecontrolled by a certain rule. Specifically, the certain rule may bedetermined by the amplitude of the movement vector, setting by a user,or external environment (e.g., surrounding brightness or temperature),or a combination of the above.

FIG. 22B illustrates a method in which the amount of movement of animage is detected from image data included in a plurality of frames, andan intermediate image between the images included in the plurality offrames is used as an interpolation image, and the display frame rate ismade higher than the input frame rate. In this embodiment mode, thedriving method illustrated in FIG. 22B is referred to as double-framerate driving.

In FIG. 22C, the period T_(in) represents a cycle of input image data.An image 2221 represents the p-th image. An image 2222 represents the(p+1)th image. An image 2223 represents the (p+2)th image. Note that the(p+1)th image 2222 may be an image obtained by controlling luminance ofthe p-th image 2221 by a certain rule. Specifically, the certain rulemay be determined by the amplitude of the movement vector, setting by auser, or external environment (e.g., surrounding brightness ortemperature), or a combination of the above.

FIG. 22C illustrates a method in which a dark image or a black image isused as an interpolation image to make pseudo impulse type display. Inthis embodiment mode, the driving method illustrated in FIG. 22C isreferred to as black frame insertion driving.

In FIG. 22D, the period T_(in) represents a cycle of input image data.An image 2231 represents the p-th image. An image 2232 represents the(p+1)th image. An image 2233 represents the (p+2)th image. Note that the(p+1)th image 2232 may be an image formed in accordance with image dataof the p-th image 2231 by a certain rule. Specifically, the certain rulemay be determined by the amplitude of the movement vector, setting by auser, or external environment (e.g., surrounding brightness ortemperature), or a combination of the above.

In FIG. 22D, the gray scale level of the p-th image 2231 is made higherand the (p+1)th image 2232 is displayed as an interpolation image for aportion having saturated luminance to interpolate the gray scale of thep-th image 2231, which leads to pseudo impulse type display. In thisembodiment mode, the driving method illustrated in FIG. 22D is referredto as time-division gray scale control driving.

In FIG. 22E, the period T_(in) represents a cycle of input image data.An image 2241 represents the p-th image. An image 2243 represents the(p+2)th image.

FIG. 22E illustrates hold-type driving in which a basic image is beingdisplayed during a cycle of input image data.

In FIGS. 22A to 22E, the amount of movement of a region whose positionis changed from frame to frame in the basic images varies. In otherwords, the amount of movement of the image illustrated in FIG. 22A(luminance control double-frame rate driving) is the largest, and isfollowed by that of the image illustrated in FIG. 22B (double-frame ratedriving), that of the image illustrated in FIG. 22C (black frameinsertion driving), that of the image illustrated in FIG. 22D(time-division gray scale control driving), and that of the imageillustrated in FIG. 22E (hold-type driving). In such a manner, inaccordance with the amount movement of the image, the driving method maybe changed for display. Accordingly, an appropriate driving method forthe amount of movement of the image can be selected; thus, motion blurcan be effectively reduced, and increase in heat generation due toincrease in power consumption and processing amount can be reduced.Further, flickers in an image with small movement can be reduced.

Next, a flow chart for selection of a driving method based on movementof an image and surrounding brightness is described with reference toFIG. 23.

After the start of the flow chart, whether to detect surroundingbrightness or not is selected in Step 1. When the surrounding brightnessis detected, the operation proceeds to Step 2. When the surroundingbrightness is not detected, the operation proceeds to Step 6. Note thatthe case where the surrounding brightness is not detected includes thecase where a device does not have a means to detect surroundingbrightness.

In Step 2, the surrounding brightness is detected. Then, the operationproceeds to Step 3.

In Step 3, whether the brightness detected in Step 2 is equal to orlower than a predetermined brightness threshold value is determined.When the brightness is equal to or lower than the brightness thresholdvalue, the operation proceeds to Step 4. When the brightness is higherthan the brightness threshold value, the operation proceeds to Step 5.Note that the brightness threshold value may be stored in a memory inthe device. Further, the brightness threshold value may be designated bya user.

In Step 4, when the device includes a backlight like a liquid crystaldisplay device, a backlight blinking mode is selected. Then, theoperation proceeds to Step 5. When the device does not include abacklight, the operation directly proceeds to Step 5. Note that thebacklight blinking mode may be a mode in which luminance of the wholebacklight is increased or decreased at one time or a mode in whichluminance of a part of the backlight is sequentially increased ordecreased. When the backlight blinking mode is selected and the maximumluminance of the backlight is the same, an average luminance becomessmall; thus, the backlight is darkened. However, when surroundingbrightness is equal to or lower than the threshold value, darkness ofthe backlight makes recognition of display more easily, black blurringcan be suppressed, and power consumption can be reduced.

In Step 5, when the device includes a backlight like a liquid crystaldisplay device, backlight output is determined. Then, the operationproceeds to Step 6. Meanwhile, when the device does not include abacklight, the operation directly proceeds to Step 6, or alternatively,the maximum luminance is determined by the detected brightness, andthereafter, the operation directly proceeds to Step 6. Note that thebacklight output is preferably reduced as the surrounding brightness issmaller. Accordingly, power consumption can be reduced while blackblurring is reduced.

In Step 6, a movement vector ∈ is detected. Thereafter, the operationproceeds to Step 7. Note that in this embodiment mode, the movementvector ∈ is regarded as a scalar quantity.

In Step 6, the movement vector ∈ detected from an image may be onemovement vector or a vector obtained from a plurality of movementvectors. For example, a plurality of movement vectors are detected, anda movement vector ∈ to be used for selecting a driving method may beobtained from the size and the number of the detected movement vectors.

In Step 7, whether the movement vector ∈ is larger than a firstthreshold value ∈1 of the movement vector is determined. When themovement vector ∈ satisfies this condition, the operation proceeds toStep 12. When the movement vector ∈ does not satisfy this condition, theoperation proceeds to Step 8.

In Step 8, whether the movement vector ∈ is larger than a secondthreshold value ∈2 of the movement vector and equal to or smaller thanthe first threshold value ∈1 is determined. When the movement vector ∈satisfies this condition, the operation proceeds to Step 13. When themovement vector ∈ does not satisfy this condition, the operationproceeds to Step 9.

In Step 9, whether the movement vector ∈ is larger than a thirdthreshold value ∈3 of the movement vector and equal to or smaller thanthe second threshold value ∈2 is determined. When the movement vector ∈satisfies this condition, the operation proceeds to Step 14. When themovement vector ∈ does not satisfy this condition, the operationproceeds to Step 10.

In Step 10, whether the movement vector ∈ is larger than a fourththreshold value ∈4 of the movement vector and equal to or smaller thanthe third threshold value ∈3 is determined. When the movement vector ∈satisfies this condition, the operation proceeds to Step 15. When themovement vector ∈ does not satisfy this condition, the operationproceeds to Step 11.

In Step 11, hold-type driving (FIG. 22E) is selected. Then, theoperation proceeds to Step 16.

In Step 12, luminance control double-frame rate driving (FIG. 22A) isselected. Then, the operation proceeds to Step 16.

In Step 13, double-frame rate driving (FIG. 22B) is selected. Then, theoperation proceeds to Step 16.

In Step 14, black frame insertion driving (FIG. 22C) is selected. Then,the operation proceeds to Step 16.

In Step 15, time-division gray scale control driving (FIG. 22D) isselected. Then, the operation proceeds to Step 16.

In Step 16, the selected driving method is held for a certain period.Then, the operation is finished.

Note that in Step 16, a period for holding the driving method can beselected as appropriate. For example, the driving method may be switchedfor each frame based on the movement vector detected in each frame, maybe switched every several seconds, or may be switched every severalminutes, or the period for holding the driving method may be determinedbased on a displayed mode set by a user or the like.

As described above, when a driving method is selected in accordance withthe flow chart shown in FIG. 23, a driving method appropriate for theamount of movement of an image can be selected. Thus, motion blur can beeffectively reduced, and increase in heat generation due to increase inpower consumption and processing amount can be reduced. Further,flickers in an image with small movement can be reduced.

In this embodiment mode, four threshold values of the movement vectorsare set, and one of five driving methods is selected based on thethreshold values. However, a driving method to be selected is notlimited to the above-described driving methods, and various drivingmethods can be used. Further, if the number of driving methods which canbe selected is large, a driver circuit becomes complicated, and thusprocessing is also complicated; therefore, the number of driving methodsis preferably five or less. Thus, manufacturing cost and powerconsumption can be reduced.

A magnitude relation of the threshold values ∈1 to ∈4 of the movementvectors may satisfy 0<∈4<∈3<∈2<∈1.

Note that although this embodiment mode is described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, even more drawings can be formed by combining each part withanother part in the above-described drawings.

Similarly, the contents (or part of the contents) described in eachdrawing of this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, even more drawings can beformed by combining each part with part of another embodiment mode inthe drawings of this embodiment mode.

This embodiment mode shows an example of an embodied case of thecontents (or part of the contents) described in another embodiment mode,an example of slight transformation thereof, an example of partialmodification thereof, an example of improvement thereof, an example ofdetailed description thereof, an application example thereof, an exampleof related part thereof, or the like. Therefore, the contents describedin another embodiment mode can be freely applied to, combined with, orreplaced with this embodiment mode.

Embodiment Mode 3

In this embodiment mode, a method for improving quality of moving imagesby dividing one frame into two or more subframes and using some of thetwo or more subframes mainly for display images (a bright image) and theothers mainly for reducing afterimages of moving images (a dark image)is described.

Difference between a black image and a dark image is described. Theblack image is an image where all of the pixels for forming an image arein a non-lighting state or a non-transmitting state, and is just an inkyblack image. On the other hand, the dark image is an image formed whenmost of the pixels for forming an image emit light with relatively lowluminance. In other words, the dark image is an image where the totalamount of light emission of all the pixels for forming an image issmaller than that of a corresponding bright image. In accordance withthis definition, the black image is used as the dark image in somecases.

Next, integrated luminance is described. In general, an image formed asa collection of pixels arranged in a display device is not alwaysperceived by a human as it is.

First, when the size of pixels is sufficiently small, human eyes cannotdistinguish pixels which are dispersedly arranged from pixels spatiallyadjacent to each other. For example, when light emission colors of theadjacent pixels are different from each other, difference of the lightemission colors is not perceived, and the different colors are perceivedas a mixed color of the adjacent pixels. This characteristic is referredto as juxtaposition color mixture and enables a color image to bedisplayed. Further, when adjacent pixels have different luminances, anintermediate value of the luminances of the adjacent pixels isperceived. Examples of techniques of expressing intermediate luminanceby utilizing this characteristic include gray scale interpolationtechniques such as dithering and error diffusion, and an area gray scalemethod which expresses a gray scale depending on the area of alight-emitting region.

Secondly, when the time in which pixels emit light is sufficiently shortand the pixels emit light a plurality of times with temporal dispersion,human eyes cannot distinguish difference of luminances temporally closeto each other. For example, when light emission with high luminance andlight emission with low luminance are continuously performed, human eyesperceive that the pixel emits light with the intermediate luminance. Atechnique which utilizes this characteristic to express the intermediateluminance is referred to as a time gray scale method. Further, whenlight emission colors are different temporally close to each other, thelight emission color of the pixel is perceived as a mixed color of thecolors temporally close to each other. Examples of a technique ofdisplaying color images by utilizing this characteristic include a fieldsequential method.

Such a phenomenon in which human eyes cannot distinguish difference ofluminances temporally close to each other when light is emitted aplurality of times with temporal dispersion is related to time-frequencycharacteristic of human eyes. Human eyes do not perceive change ofluminance at higher frequency than a given critical value, and is seemsthat light continue to be emitted with constant luminance. At this time,the luminance which is perceived by the human eyes depends on a valueobtained by integrating the luminance by time (integrated luminance).

On the other hand, when luminance changes at frequency that is lowerthan or equal to a given critical value, human eyes perceive the changein luminance as flickers as it is. The critical value depends onluminance and is approximately several tens of Hz (a cycle is ten toseveral tens of milliseconds). That is, integrated luminance is a valueobtained by integrating luminance by time in the time range up toseveral tens of milliseconds in which change in luminance is notperceived by human eyes. Next, formulation of the integrated luminancewhen one frame is divided into a plurality of subframes is describedwith reference to FIGS. 2A and 2B. A solid line in FIG. 2A shows anexample of temporal change of luminance of a pixel in one frame when oneframe is divided into two subframes.

In FIG. 2A, the length of one frame period is denoted by T_(in), thelength of a first subframe is denoted by T₁, the length of a secondsubframe is denoted by T₂, average luminance of a pixel in the firstsubframe period is denoted by X₁, and average luminance of the pixel inthe second subframe period is denoted by X₂. Integrated luminance in thefirst subframe period is the product of T₁ and X₁. Similarly, integratedluminance in the second subframe period is the product of T₂ and X₂.

Note that temporal change of luminance is not like the solid line inFIG. 2A in some cases, due to characteristics of a device used as adisplay device. For example, in the case of a display device usingliquid crystals, luminance changes gently as shown by a dashed line inFIG. 2A. In such a case, the integrated luminance is precisely definedby time integration of luminance; however, in this embodiment mode, theintegrated luminance is defined by the product of average luminance anda subframe period for simplicity. Accordingly, luminance in eachsubframe period is not necessarily constant.

FIG. 2B shows an example of distribution of integrated luminance in oneframe period with respect to gray scale levels to be displayed. Thehorizontal axis represents a gray scale level, and the vertical axisrepresents integrated luminance in one frame period. FIG. 2B shows thecase where display at the gray scale level of 0 to the gray scale levelof 255 is performed. Note that the cases of the gray scale level of 5 tothe gray scale level of 251 are not shown in the drawing. In each grayscale, a shaded portion represents integrated luminance in the firstsubframe period, and a white portion represents integrated luminance inthe second subframe period.

As described above, integrated luminance in one frame period can beexpressed as the sum of the integrated luminance in the first subframeperiod and the integrated luminance in the second subframe period. Thedistribution of the integrated luminance can be set individuallydepending on a gray scale level to be displayed.

The number of subframe periods by which one frame period is divided isnot particularly limited as long as it is an integer equal to or morethan 2. When formulated, this can be expressed as below. That is, whenone frame period is divided into n subframe periods (n is an integerequal to or more than 2), average luminance of a display element in thei-th subframe period (i is an integer equal to or more than 1 and equalto less than n) is denoted by Xi and the length of the i-th subframeperiod is denoted by Ti, integrated luminance Y which is obtained bytime-integrating a function X(t) of luminance related to time by the oneframe period can be expressed as Formula 3.

$\begin{matrix}{Y = {\sum\limits_{i = 1}^{n}{X_{i}T_{i}}}} & \lbrack {{Formula}\mspace{14mu} 3} \rbrack\end{matrix}$

Note that the length Ti of the i-th subframe period is preferablyapproximately the same in every subframe period. This is because aperiod in which image data is written to a pixel (an address period) canbe longest when the length of all the subframe periods is the same. Whenthe address period is long, operating frequency of a peripheral drivercircuit of the display device can be reduced; thus, power consumptioncan be reduced. Further, yield of the display device is improved.However, the invention is not limited to this structure, and Ti may varydepending on each subframe period. For example, when the length of asubframe period for displaying a bright image is longer, averageluminance of a backlight unit can be increased without increase in powerconsumption. Moreover, power consumption can be reduced without changeof average luminance of the backlight unit. In other words, emissionefficiency can be improved. Further, when the length of a subframeperiod for displaying a dark image is longer, quality of moving imagescan be significantly improved.

In this embodiment mode, the case is described in which the divisionnumber n of subframes is 2 and the length of each subframe period is thesame. A subframe period which is the first half of one frame period isreferred to as 1SF, and a subframe period which is the latter half ofone frame period is referred to as 2SF.

Each of FIGS. 1A and 1B is a graph for showing a distribution method ofluminance to two subframe periods with respect to a gray scale to bedisplayed, in this embodiment mode. FIG. 1A shows the case whereluminance in 2SF is higher than luminance in 1SF, and FIG. 1B shows thecase where luminance in 1SF is higher than luminance in 2SF.

First, description is made with reference to FIG. 1A. In FIG. 1A, thehorizontal axis represents time, and the vertical solid lines representboundaries of frames. The vertical dashed lines represent boundaries ofsubframes. The vertical axis represents luminance. That is, FIG. 1Ashows change of luminance of a pixel with respect to time over fiveframes when the luminance is increased along with time.

The degree of gray scale in each frame is represented below thehorizontal axis. That is, FIG. 1A shows change of luminance of a pixelwith respect to time in the case where a minimum gray scale is displayedfirst, and then, halftone on the lower gray scale level side, halftoneof the intermediate level, halftone on the higher gray scale level side,and a maximum gray scale are displayed in this order.

Although quality of moving images is improved by inserting a blackimage, an aspect of a driving method of a display device which isdescribed in this embodiment mode is that quality of moving images isimproved by inserting a dark image which is close to black but is not ablack image. That is, one frame period is divided into two subframeperiods 1SF and 2SF and light is emitted so that luminance of 1SF islower than luminance of 2SF when a maximum gray scale is to bedisplayed; thus, improvement in quality of moving images is realized,and luminance in one frame period is kept constant.

As for a method for expressing gray scales, first, in the range of theminimum gray scale to the halftone of the intermediate degree, the grayscales are expressed by the level of the luminance in 2SF. Then, afterthe luminance in 2SF reaches maximum value L_(max2), the luminance in2SF is fixed to L_(max2) and the gray scales are expressed by the levelof the luminance in 1SF Then, when the maximum gray scale is to beexpressed, luminance L_(max1) in 1SF is preferably lower than L_(max2)because quality of moving images can be improved.

That is, when time in which the luminance is maintained (hold time) isreduced even near the maximum gray scale, afterimages are reduced in therange of all the gray scale levels; thus, quality of moving images canbe favorable. Further, a dark image, instead of a black image, isdisplayed in 1SF when the maximum gray scale is displayed; thus,luminance of L_(max1) can be reduced. Accordingly, power consumption canbe reduced.

L_(max1) is preferably 90% or less of L_(max2), and more preferably 60%or less of L_(max2) in order to improve quality of moving images.Further, in order to increase L_(max1) and suppress the maximumluminance in one frame so as to reduce power consumption, L_(max1) ispreferably 50% or more of L_(max2). That is, when a dark image isinserted in 1SF, L_(max1) is preferably in the range represented asfollows: (1/2)L_(max2)<L_(max1)<(9/10)L_(max2), and more preferably(1/2)L_(max2)<L_(max1)<(3/5)L_(max2).

Note that the length of one frame period is preferably less than orequal to 1/60 seconds so that a flicker does not easily occur. However,the shorter the length of one frame period is, the higher the operatingfrequency of a peripheral driver circuit becomes, which leads toincrease in power consumption. Therefore, the length of one frame periodis preferably in the range of 1/120 to 1/60 seconds.

Next, the case where luminance in 1SF is higher than luminance in 2SF isdescribed with reference to FIG. 1B. In FIG. 1B, the horizontal axisrepresents time, and the vertical solid lines represent boundaries offrames. The vertical dashed lines represent boundaries of subframes. Thevertical axis represents luminance. That is, FIG. 1B shows change ofluminance of a pixel with respect to time over five frames. Although theluminance of 1SF is lower than that of 2SF in FIG. 1A, the invention isnot limited to this. That is, as shown in FIG. 1B, one frame period isdivided into two subframe periods 1SF and 2SF, and light is emitted sothat luminance of 2SF is lower than luminance of 1SF when a maximum grayscale is displayed; thus, improvement in quality of moving images can berealized. In this manner, it is possible to reverse the order of 1SF and2SF.

Note that although this embodiment mode is described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, even more drawings can be formed by combining each part withanother part in the above-described drawings.

Similarly, the contents (or part of the contents) described in eachdrawing of this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, even more drawings can beformed by combining each part with part of another embodiment mode inthe drawings of this embodiment mode.

This embodiment mode shows an example of an embodied case of thecontents (or part of the contents) described in another embodiment mode,an example of slight transformation thereof, an example of partialmodification thereof, an example of improvement thereof, an example ofdetailed description thereof, an application example thereof, an exampleof related part thereof, or the like. Therefore, the contents describedin another embodiment mode can be freely applied to, combined with, orreplaced with this embodiment mode.

Embodiment Mode 4

In this embodiment mode, another example of a method in which one frameis divided into a plurality of subframes, and some of the plurality ofsubframes are used mainly for displaying images (bright images) and theothers are used mainly for reducing afterimages of moving images (darkimages), which is described in Embodiment Mode 3, is described.

When images to be displayed are divided into bright images and darkimages, there are several methods with different ways of distribution ofluminance which is needed to express a gray scale of an image to bedisplayed to a plurality of subframes. In order to describe thesemethods, in this embodiment mode, graphs each of which horizontal axisrepresents a gray scale level and vertical axis represents integratedluminance are employed. Each graph shows a relation between integratedluminance and a gray scale level in 1SF, a relation between integratedluminance and a gray scale level in 2SF, and a relation betweenintegrated luminance and a gray scale level of the sum of 1SF and 2SF.

First, one mode of this embodiment mode is described with reference toFIG. 3A. FIG. 3A shows an example of a method for distributing the totalintegrated luminance in one frame to 1SF and 2SF. Further, a table underthe graph shows a feature of each subframe briefly. A subframe which isdescribed as having a constant slope in the table means that change ofintegrated luminance with respect to a gray scale level is constant.That is, in the mode shown in FIG. 3A, change of the integratedluminance with respect to a gray scale level of 2SF is constant.Although FIG. 3A shows the case where the slope is positive, the slopemay be 0 or negative as well. Moreover, when a subframe is described as(the sum−xSF) in the table, the integrated luminance of the subframedepends on the integrated luminance of another subframe. A variety ofsubframes such as 1SF or 2SF may correspond to xSF. That is, in the modeshown in FIG. 3A, the integrated luminance of 1SF is a value obtained bysubtracting the integrated luminance of 2SF from the total luminance.Here, the total luminance is separately determined, and in thisembodiment mode, it is shown by a curve which is convex downward. Thisis the case where gamma correction is performed in consideration ofcharacteristics of human eyes. Note that the total luminance may belinear with respect to a gray scale level, a curve which is convexupward, or a combination of a line segment and a curve. Furthermore, amechanism in which the total luminance and the gamma correction areswitched depending on a display image or a mechanism in which the totalluminance and the gamma correction can be controlled by a user may beprovided.

In the mode shown in FIG. 3A, since the change of the integratedluminance with respect to the gray scale level of 2SF is constant, imageprocessing and applied voltage are simplified and a load of a peripheraldriver circuit is reduced. In the mode shown in FIG. 3A, 1SF and 2SF canbe exchanged as shown in FIGS. 1A and 1B, and even when characteristicsof 1SF and 2SF are exchanged, a similar effect can be obtained. Notethat the luminance in 1SF is higher than the luminance in 2SF; however,the invention is not limited to this. The luminance in 1SF may be lowerthan the luminance in 2SF. Note that when the total luminance isnonlinear, the luminance in 2SF is preferably lower than that in 1SFbecause the gray scale level can be controlled more easily.

FIG. 3B shows an example of a method for distributing the totalintegrated luminance in one frame to 1SF and 2SF. As in FIG. 3B, asubframe which is described as having a constant ratio in a table underthe graph means that the case where an integrated luminance ratiobetween 1SF and 2SF is equal at each gray scale level. That is, the modeshown in FIG. 3B shows the case where the ratio between the integratedluminance of 1SF and the integrated luminance of 2SF is equal in anygray scale level. Note that a value of the ratio in this case (the levelof the smaller luminance with respect to the larger luminance) ispreferably greater than 0.5 and less than 1. Accordingly, motion blurcan be efficiently reduced. In the case of having a characteristic ofthe constant ratio, each of the two subframes may have a characteristicof the constant ratio. In other words, it may be said that there is nocase where one of the subframes has a constant ratio and the other doesnot. In the mode shown in FIG. 3B, 1SF and 2SF can be exchanged, andeven when the characteristics of 1SF and 2SF are exchanged, a similareffect can be obtained. Note that the luminance in 1SF is higher thanthe luminance in 2SF; however, the invention is not limited to this. Theluminance in 1SF may be lower than the luminance in 2SF. Note that whenthe total luminance is nonlinear, the luminance in 2SF is preferablylower than the luminance in 1SF because the gray scale level can becontrolled more easily.

Next, another mode of this embodiment mode is described with referenceto FIGS. 4A to 4F. FIGS. 4A to 4F each show an example of a method fordistributing the total integrated luminance in one frame to 1SF and 2SFin the case where gray scale levels which can be displayed are dividedinto a plurality of regions, for example, two regions and each subframecan have different characteristics in each region. In this embodimentmode, for explanation, each of the regions is referred to as a region 1,a region 2, . . . in ascending order of gray scale level.

In the description below, “a value of integrated luminance is continuousat the boundary of regions” is defined as follows. Of two adjacent grayscale levels which are separated by the boundary of regions, when a grayscale level which belongs to a region on the lower gray scale level sideis a boundary gray scale level (low), a gray scale level which belongsto a region on the higher gray scale level side is a boundary gray scalelevel (high), and an absolute value of luminance difference between theboundary gray scale level (high) and the boundary gray scale level (low)is boundary luminance difference; “a value of integrated luminance iscontinuous at the boundary of regions” means that the boundary luminancedifference is less than or equal to a certain value Δx.

Here, the value of Δx can be a variety of values depending on theluminance in the boundary gray scale level (high), the luminance in theboundary gray scale level (low), and the like. However, it can bedetermined from a standpoint of continuity of gray scale level-luminancecharacteristics seen from the human eyes (i.e., whether or not an imagecorresponding to the gray scale level-luminance characteristics isdisplayed smoothly on the boundary of the regions). Specifically, whenan absolute value of difference between the luminance at the boundarygray scale level (low) and the luminance which is lower than theboundary gray scale level (low) by 1 is a first neighborhood boundaryluminance difference (low), Δx is preferably approximately twice thefirst neighborhood boundary luminance difference (low).

In this embodiment mode and other embodiment modes, description is madeon the assumption that Δx is twice the first neighborhood boundaryluminance difference (low).

FIG. 4A shows an example of a method for distributing the totalintegrated luminance in one frame to 1SF and 2SF. A table under thegraph shows a feature of each subframe briefly. A subframe described ashaving a constant slope (continuous) (positive slope) in a column of theregion 2 in the table means that change of the integrated luminance withrespect to the gray scale level is constant, a value of the integratedluminance at the boundary with the adjacent region on the lower grayscale level side (the region 1) is continuous, and change of theintegrated luminance with respect to the gray scale level in this regionhas a positive sign. With such features, luminance difference between1SF and 2SF in the maximum gray scale is reduced, so that flickers indisplaying an image can be reduced.

FIG. 4B shows an example of a method for distributing the totalintegrated luminance in one frame to 1SF and 2SF. A table under thegraph shows a feature of each subframe briefly. A subframe described ashaving a constant slope (continuous) (slope 0) in a column of the region2 in the table means that change of the integrated luminance withrespect to the gray scale level is constant, a value of the integratedluminance at the boundary with the adjacent region on the lower grayscale level side (the region 1) is continuous, and change of theintegrated luminance with respect to the gray scale level in this regionis 0. With such features, image processing and applied voltage aresimplified, and a load of a peripheral driver circuit is reduced.

FIG. 4C shows an example of a method for distributing the totalintegrated luminance in one frame to 1SF and 2SF. A table under thegraph shows a feature of each subframe briefly. A subframe described ashaving a constant slope (continuous) (negative slope) in a column of theregion 2 in the table means that change of the integrated luminance withrespect to the gray scale level is constant, a value of the integratedluminance at the boundary with the adjacent region on the lower grayscale level side (the region 1) is continuous, and change of theintegrated luminance with respect to the gray scale level in this regionhas a negative sign. With such features, luminance difference between1SF and 2SF in the maximum gray scale is increased, so that motion blurcan be efficiently reduced.

In the modes shown in FIGS. 4A, 4B, and 4C, 1SF and 2SF can beexchanged, and even when the characteristics of 1SF and 2SF areexchanged, a similar effect can be obtained. Although the luminance in1SF is higher than the luminance in 2SF, the invention is not limited tothis. The luminance in 1SF may be lower than the luminance in 2SF. Notethat when the total luminance is nonlinear, the luminance in 2SF ispreferably lower than the luminance in 1SF because the gray scale levelcan be controlled more easily. Further, a magnitude relation of theluminance between 1SF and 2SF may be exchanged separately in eachregion. A region or regions in which the magnitude relation of luminanceis exchanged may be only the region 1, only the region 2, or the region1 and the region 2, for example.

In such a manner, when gray scales which can be displayed are dividedinto a plurality of regions, change of integrated luminance with respectto a gray scale level (a value of slope) in each region can be a varietyof values. However, as shown in FIG. 4D, the value of slope ispreferably smaller than a slope of a tangent line of the total value ofintegrated luminance at the boundary of the regions. That is, when aslope of a tangent line of the total value of integrated luminance atthe boundary of the regions is θmax, the value θ in the regions ispreferably in the range of (−θmax<θ<θmax) (a hatched region in FIG. 4D).When θ is within the range, a phenomenon in which when change ofintegrated luminance with respect to a gray scale level is sharp, thegray scale level at the boundary of the regions is intensified and anunnatural contour is generated can be reduced.

As a method for reducing a phenomenon in which when change of integratedluminance with respect to a gray scale level is sharp, the gray scalelevel at the boundary of the regions is intensified and an unnaturalcontour is generated, methods shown in FIGS. 4E and 4F can be used aswell as the method shown in FIG. 4D. A feature of each region in FIGS.4E and 4F is the same as the mode shown in FIG. 4B, and a gray scalelevel at the boundary of the regions is different. When a plurality ofluminance distribution modes in which a gray scale level at the boundaryof the regions varies are prepared and switched as needed, a phenomenonin which the gray scale level at the boundary of the regions isintensified and an unnatural contour is generated can be reduced. Such amethod can be applied to a variety of luminance distribution modes,without being limited to the mode shown in FIG. 4B.

As for a method for switching a plurality of luminance distributionmodes, the luminance distribution modes may be switched per frame, forexample. Accordingly, a phenomenon in which an unnatural contour isgenerated can be efficiently reduced. Alternatively, the luminancedistribution modes may be switched in accordance with an image to bedisplayed. At this time, when a threshold value exists in gray scalelevel distribution of the image, the boundary of the regions ispreferably set near the threshold value. For example, in the case of abright image with little distribution of gray scale levels less than orequal to the gray scale level of 100, the boundary of the regions ispreferably set near the gray scale level of 100. Similarly, also in thecase of a dark image with little distribution of gray scale levelsgreater than or equal to the gray scale level of 100, the boundary ofthe regions is preferably set near the gray scale level of 100.Accordingly, in an image to be displayed, gray scale levels which crossnear the threshold value are reduced, so that a phenomenon in which thegray scale level at the boundary of the regions is intensified and thusan unnatural contour is generated can be reduced. Note that thethreshold value may be set depending on the brightness of an image. Forexample, the boundary of the regions may be set on the higher gray scalelevel side in the case of a generally dark image, and the boundary ofthe regions may be set on the lower gray scale level side in the case ofa generally bright image. Accordingly, in an image to be displayed, grayscale levels which cross near the threshold value are reduced, so that aphenomenon in which the gray scale level at the boundary of the regionsis intensified and thus an unnatural contour is generated can bereduced. Note that the method of switching the luminance distributionmode in accordance with an image to be displayed can be applied to avariety of luminance distribution modes, without being limited to modeswith different boundaries of the regions.

Next, another mode of this embodiment mode is described with referenceto FIGS. 5A to 5F. FIGS. 5A to 5F each show an example of a method fordistributing the total integrated luminance in one frame to 1SF and 2SFin the case where gray scales which can be displayed are divided into aplurality of regions, for example, two regions and each subframe canhave different characteristics in each region. In particular, the casewhere change of integrated luminance with respect to a gray scale of oneof subframes is constant in both of the two regions is described.

FIG. 5A shows an example of a method for distributing the totalintegrated luminance in one frame to 1SF and 2SF. A table under thegraph shows a feature of each subframe briefly. A feature of 2SF in aregion 1 is that change of integrated luminance with respect to a grayscale level is constant. A value of the slope may be positive, 0, ornegative. A feature of 1SF in the region 1 is that the luminance dependson the total luminance and the luminance of 2SF. A feature of 1SF in aregion 2 is that change of integrated luminance with respect to a grayscale level is constant and a value of integrated luminance iscontinuous at the boundary with an adjacent region on the lower grayscale level side (the region 1). A value of the slope may be positive,0, or negative. A feature of 2SF in the region 2 is that the luminancedepends on the total luminance and the luminance of 1SF. With suchfeatures, image processing and applied voltage are simplified, and aload of a peripheral driver circuit is reduced. Moreover, a phenomenonin which an unnatural contour is generated can be reduced. Further,since the maximum luminance in 1SF and 2SF can be lowered, powerconsumption can be reduced.

FIG. 5B shows an example of a method for distributing the totalintegrated luminance in one frame to 1SF and 2SF. A table under thegraph shows a feature of each subframe briefly. A feature of 2SF in aregion 1 is that change of integrated luminance with respect to a grayscale level is constant. A value of the slope may be positive, 0, ornegative. A feature of 1SF in the region 1 is that the luminance dependson the total luminance and the luminance of 2SF. A feature of 2SF in aregion 2 is that change of integrated luminance with respect to a grayscale level is constant and a value of integrated luminance changesdiscontinuously toward a larger value of integrated luminance at theboundary with an adjacent region on the lower gray scale level side (theregion 1). A value of the slope may be positive, 0, or negative. Afeature of 1SF in the region 2 is that the luminance depends on thetotal luminance and the luminance of 2SF. With such features, luminancedifference between 1SF and 2SF in the maximum gray scale is reduced, sothat flickers in displaying an image can be reduced. Further, since theluminance change of 2SF is simplified, image processing and appliedvoltage are simplified, and a load of a peripheral driver circuit isreduced. In particular, capacitance of a memory element can be reducedwhen overdriving is performed.

FIG. 5C shows an example of a method for distributing the totalintegrated luminance in one frame to 1SF and 2SF. A table under thegraph shows a feature of each subframe briefly. A feature of 2SF in aregion 1 is that change of integrated luminance with respect to a grayscale level is constant. A value of the slope may be positive, 0, ornegative. A feature of 1SF in the region 1 is that the luminance dependson the total luminance and the luminance of 2SF. A feature of 1SF in aregion 2 is that change of integrated luminance with respect to a grayscale level is constant and a value of integrated luminance changesdiscontinuously toward a smaller value of integrated luminance at theboundary with an adjacent region on the lower gray scale level side (theregion 1). A value of the slope may be positive, 0, or negative. Afeature of 2SF in the region 2 is that the luminance depends on thetotal luminance and the luminance of 1SF. With such features, luminancedifference between 1SF and 2SF in the maximum gray scale is reduced, andflickers in displaying an image can be reduced.

FIG. 5D shows an example of a method for distributing the totalintegrated luminance in one frame to 1SF and 2SF. A table under thegraph shows a feature of each subframe briefly. A feature of 2SF in aregion 1 is that change of integrated luminance with respect to a grayscale level is constant. A value of the slope may be positive, 0, ornegative. A feature of 1SF in the region 1 is that the luminance dependson the total luminance and the luminance of 2SF. A feature of 2SF in aregion 2 is that change of integrated luminance with a respect to grayscale level is constant and a value of integrated luminance iscontinuous at the boundary with an adjacent region on the lower grayscale level side (the region 1). A value of the slope may be positive,0, or negative. A feature of 1SF in the region 2 is that the luminancedepends on the total luminance and the luminance of 2SF. With suchfeatures, image processing and applied voltage are simplified, and aload of a peripheral driver circuit is reduced. Further, a phenomenon inwhich an unnatural contour is generated can be reduced. Moreover, sincethe maximum luminance in 1SF and 2SF can be lowered, power consumptioncan be reduced.

FIG. 5E shows an example of a method for distributing the totalintegrated luminance in one frame to 1SF and 2SF. A table under thegraph shows a feature of each subframe briefly. A feature of 2SF in aregion 1 is that change of integrated luminance with respect to a grayscale level is constant. A value of the slope may be positive, 0, ornegative. A feature of 1SF in the region 1 is that the luminance dependson the total luminance and the luminance of 2SF. A feature of 1SF in aregion 2 is that change of integrated luminance with respect to a grayscale level is constant and a value of integrated luminance changesdiscontinuously toward a larger value of integrated luminance at theboundary with an adjacent region on the lower gray scale level side (theregion 1). A value of the slope may be positive, 0, or negative. Afeature of 2SF in the region 2 is that the luminance depends on thetotal luminance and the luminance of 1SF. With such features, luminancedifference between 1SF and 2SF in the maximum gray scale is increased,and motion blur can be efficiently reduced.

FIG. 5F shows an example of a method for distributing the totalintegrated luminance in one frame to 1SF and 2SF. A table under thegraph shows a feature of each subframe briefly. A feature of 2SF in aregion 1 is that change of integrated luminance with respect to a grayscale level is constant. A value of the slope may be positive, 0, ornegative. A feature of 1SF in the region 1 is that the luminance dependson the total luminance and the luminance of 2SF. A feature of 2SF in aregion 2 is that change of integrated luminance with respect to a grayscale level is constant and a value of integrated luminance changesdiscontinuously toward a smaller value of integrated luminance at theboundary with an adjacent region on the lower gray scale level side (theregion 1). A value of the slope may be positive, 0, or negative. Afeature of 1SF in the region 2 is that the luminance depends on thetotal luminance and the luminance of 2SF. With such features, luminancedifference between 1SF and 2SF in the maximum gray scale is increased,and motion blur can be efficiently reduced.

In the modes shown in FIGS. 5A to 5F, 1SF and 2SF can be exchanged, andeven when the characteristics of 1SF and 2SF are exchanged, a similareffect can be obtained. Although the luminance in 1SF is higher than theluminance in 2SF, the invention is not limited to this. The luminance in1SF may be lower than the luminance in 2SF. Note that when the totalluminance is nonlinear, the luminance in 2SF is preferably lower thanthe luminance in 1SF because the gray scale level can be controlled moreeasily. A magnitude relation of luminance between 1SF and 2SF may beexchanged. Further, the magnitude relation of luminance between 1SF and2SF may be exchanged separately in each region. A region or regions inwhich the magnitude relation of luminance is exchanged may be only theregion 1, only the region 2, or the region 1 and the region 2, forexample.

Next, another mode of this embodiment mode is described with referenceto FIGS. 6A to 6F. FIGS. 6A to 6F each show an example of a method fordistributing the total integrated luminance in one frame to 1SF and 2SFin the case where gray scale levels which can be displayed are dividedinto a plurality of regions, for example, two regions and each subframecan have different characteristics in each region. In particular, thecase where change of integrated luminance with respect to a gray scalelevel of one of subframes in one of the two regions is constant and anintegrated luminance ratio between 1SF and 2SF is equal in each grayscale level in the other region is described.

FIG. 6A shows an example of a method for distributing the totalintegrated luminance in one frame to 1SF and 2SF. A table under thegraph shows a feature of each subframe briefly. A feature of 1SF and 2SFin a region 1 is that an integrated luminance ratio between 1SF and 2SFis equal in each gray scale. A value of the ratio in this case (thelevel of the smaller luminance with respect to the larger luminance) ispreferably greater than 0.1 and less than 0.5. Accordingly, luminancedifference between 1SF and 2SF on the lower gray scale level side can beincreased, and motion blur can be efficiently reduced. A feature of 2SFin a region 2 is that change of integrated luminance with respect to agray scale is constant and a value of integrated luminance is continuousat the boundary with an adjacent region on the lower gray scale levelside (the region 1). A value of the slope may be positive, 0, ornegative. A feature of 1SF in the region 2 is that the luminance dependson the total luminance and the luminance of 2SF. With such features,image processing and applied voltage are simplified, and a load of aperipheral driver circuit is reduced. Further, a phenomenon in which anunnatural contour is generated can be reduced. Moreover, since themaximum luminance in 1SF and 2SF can be lowered, power consumption canbe reduced.

FIG. 6B shows an example of a method for distributing the totalintegrated luminance in one frame to 1SF and 2SF. A table under thegraph shows a feature of each subframe briefly. A feature of 1SF and 2SFin a region 1 is that an integrated luminance ratio between 1SF and 2SFis equal in each gray scale. A value of the ratio in this case (thelevel of the smaller luminance with respect to the larger luminance) ispreferably greater than 0.1 and less than 0.5. Accordingly, luminancedifference between 1SF and 2SF on the lower gray scale level side can beincreased, and motion blur can be efficiently reduced. A feature of 2SFin a region 2 is that change of integrated luminance with respect to agray scale is constant and a value of integrated luminance changesdiscontinuously toward a larger value of integrated luminance at theboundary with an adjacent region on the lower gray scale level side (theregion 1). A value of the slope may be positive, 0, or negative. Afeature of 1SF in the region 2 is that the luminance depends on thetotal luminance and the luminance of 2SF. With such features, luminancedifference between 1SF and 2SF in the maximum gray scale is reduced, andflickers in displaying an image are reduced.

FIG. 6C shows an example of a method for distributing the totalintegrated luminance in one frame to 1SF and 2SF. A table under thegraph shows a feature of each subframe briefly. A feature of 1SF and 2SFin a region 1 is that an integrated luminance ratio between 1SF and 2SFis equal in each gray scale. A value of the ratio in this case (thelevel of the smaller luminance with respect to the larger luminance) ispreferably greater than 0.1 and less than 0.5. Accordingly, luminancedifference between 1SF and 2SF on the lower gray scale level side can beincreased, and motion blur can be efficiently reduced. A feature of 2SFin a region 2 is that change of integrated luminance with respect to agray scale is constant and a value of integrated luminance changesdiscontinuously toward a smaller value of integrated luminance at theboundary with an adjacent region on the lower gray scale level side (theregion 1). A value of the slope may be positive, 0, or negative. Afeature of 1SF in the region 2 is that the luminance depends on thetotal luminance and the luminance of 2SF. With such features, luminancedifference between 1SF and 2SF in the maximum gray scale is increased,and motion blur can be efficiently reduced.

FIG. 6D shows an example of a method for distributing the totalintegrated luminance in one frame to 1SF and 2SF. A table under thegraph shows a feature of each subframe briefly. A feature of 2SF in aregion 1 is that change of integrated luminance with respect to a grayscale is constant. A value of the slope may be positive, 0, or negative.A feature of 1SF in the region 1 is that the luminance depends on thetotal luminance and the luminance of 2SF. A feature of 1SF and 2SF in aregion 2 is that an integrated luminance ratio between 1SF and 2SF isequal in each gray scale, a value of integrated luminance of 1SF changesdiscontinuously toward a smaller value, and a value of integratedluminance of 2SF changes discontinuously toward a larger value at theboundary with an adjacent region on the lower gray scale level side (theregion 1). With such features, luminance difference between 1SF and 2SFin the maximum gray scale is reduced, and flickers in displaying animage are reduced. A value of the ratio in this case (the level of thesmaller luminance with respect to the larger luminance) is preferablygreater than 0.5 and less than 1. Accordingly, motion blur can beefficiently reduced.

FIG. 6E shows an example of a method for distributing the totalintegrated luminance in one frame to 1SF and 2SF. A table under thegraph shows a feature of each subframe briefly. A feature of 2SF in aregion 1 is that change of integrated luminance with respect to a grayscale is constant. A value of the slope may be positive, 0, or negative.A feature of 1SF in the region 1 is that the luminance depends on thetotal luminance and the luminance of 2SF. A feature of 1SF and 2SF in aregion 2 is that an integrated luminance ratio between 1SF and 2SF isequal in each gray scale, and a value of integrated luminance of each of1SF and 2SF is continuous at the boundary with an adjacent region on thelower gray scale level side (the region 1). With such features, imageprocessing and applied voltage are simplified, and a load of aperipheral driver circuit is reduced. Further, a phenomenon in which anunnatural contour is generated can be reduced. A value of the ratio inthis case (the level of the smaller luminance with respect to the largerluminance) is preferably greater than 0.5 and less than 1. Accordingly,motion blur can be efficiently reduced.

FIG. 6F shows an example of a method for distributing the totalintegrated luminance in one frame to 1SF and 2SF. A table under thegraph shows a feature of each subframe briefly. A feature of 2SF in aregion 1 is that change of integrated luminance with respect to a grayscale is constant. A value of the slope may be positive, 0, or negative.A feature of 1SF in the region 1 is that the luminance depends on thetotal luminance and the luminance of 2SF. A feature of 1SF and 2SF in aregion 2 is that an integrated luminance ratio between 1SF and 2SF isequal in each gray scale, a value of integrated luminance of 1SF changesdiscontinuously toward a larger value, and a value of integratedluminance of 2SF changes discontinuously toward a smaller value at theboundary with an adjacent region on the lower gray scale level side (theregion 1). With such features, luminance difference between 1SF and 2SFin the maximum gray scale is increased, and motion blur can beefficiently reduced. A value of the ratio in this case (the level of thesmaller luminance with respect to the larger luminance) is preferablygreater than 0.5 and less than 1. Accordingly, motion blur can beefficiently reduced.

In the modes shown in FIGS. 6A to 6F, 1SF and 2SF can be exchanged, andeven when the characteristics of 1SF and 2SF are exchanged, a similareffect can be obtained. Although the luminance in 1SF is higher than theluminance in 2SF, the invention is not limited to this. The luminance in1SF may be lower than the luminance in 2SF. Note that when the totalluminance is nonlinear, the luminance in 2SF is preferably lower thanthe luminance in 1SF because the gray scale can be controlled moreeasily. Further, a magnitude relation of luminance between 1SF and 2SFmay be exchanged separately in each region. A region or regions in whichthe magnitude relation of luminance is exchanged may be only the region1, only the region 2, or the region 1 and the region 2, for example.

Next, another mode of this embodiment mode is described with referenceto FIGS. 7A and 7B. FIGS. 7A and 7B each show an example of a method fordistributing the total integrated luminance in one frame to 1SF and 2SFin the case where gray scales which can be displayed are divided into aplurality of regions, for example, two regions and each subframe canhave different characteristics in each region. In particular, the casewhere an integrated luminance ratio between 1SF and 2SF is equal in eachgray scale in the both regions is described.

FIG. 7A shows an example of a method for distributing the totalintegrated luminance in one frame to 1SF and 2SF. A table under thegraph shows a feature of each subframe briefly. A feature of 1SF and 2SFin a region 1 is that an integrated luminance ratio between 1SF and 2SFis equal in each gray scale. A value of the ratio in this case (thelevel of the smaller luminance with respect to the larger luminance) ispreferably greater than 0.1 and less than 0.5. Accordingly, luminancedifference between 1SF and 2SF on the lower gray scale level side can beincreased, and motion blur can be efficiently reduced. A feature of 1SFand 2SF in a region 2 is that an integrated luminance ratio between 1SFand 2SF is equal in each gray scale, a value of integrated luminance of1SF changes discontinuously toward a smaller value, and a value ofintegrated luminance of 2SF changes discontinuously toward a largervalue at the boundary with an adjacent region on the lower gray scalelevel side (the region 1). With such features, luminance differencebetween 1SF and 2SF in the maximum gray scale is reduced, and flickersin displaying an image are reduced. A value of the ratio in this case(the level of the smaller luminance with respect to the largerluminance) is preferably greater than 0.5 and less than 1. Accordingly,motion blur can be efficiently reduced.

FIG. 7B shows an example of a method for distributing the totalintegrated luminance in one frame to 1SF and 2SF. A table under thegraph shows a feature of each subframe briefly. A feature of 1SF and 2SFin a region 1 is that an integrated luminance ratio between 1SF and 2SFis equal in each gray scale. A value of the ratio in this case (thelevel of the smaller luminance with respect to the larger luminance) ispreferably greater than 0.1 and less than 0.5. Accordingly, luminancedifference between 1SF and 2SF on the lower gray scale level side can beincreased, and motion blur can be efficiently reduced. A feature of 1SFand 2SF in a region 2 is that an integrated luminance ratio between 1SFand 2SF is equal in each gray scale, a value of integrated luminance of1SF changes discontinuously toward a larger value, and a value ofintegrated luminance of 2SF changes discontinuously toward a smallervalue at the boundary with an adjacent region on the lower gray scalelevel side (the region 1). With such features, luminance differencebetween 1SF and 2SF in the maximum gray scale is increased, and motionblur can be efficiently reduced. A value of the ratio in this case (thelevel of the smaller luminance with respect to the larger luminance) ispreferably greater than 0.5 and less than 1. Accordingly, motion blurcan be efficiently reduced.

In the modes shown in FIGS. 7A and 7B, 1SF and 2SF can be exchanged, andeven when the characteristics of 1SF and 2SF are exchanged, a similareffect can be obtained. Although the luminance in 1SF is higher than theluminance in 2SF, the invention is not limited to this. The luminance in1SF may be lower than the luminance in 2SF. Note that when the totalluminance is nonlinear, the luminance in 2SF be lower than the luminancein 1SF because the gray scale can be controlled more easily. Further, amagnitude relation of luminance between 1SF and 2SF may be exchangedseparately in each region. A region or regions in which the magnituderelation of luminance is exchanged may be only the region 1, only theregion 2, or the region 1 and the region 2, for example.

Next, another mode of this embodiment mode is described with referenceto FIGS. 8A to 8D. FIGS. 8A to 8D each show an example of a method fordistributing the total integrated luminance in one frame to 1SF and 2SFin the case where gray scales which can be displayed are divided into aplurality of regions, for example, three regions and each subframe canhave different characteristics in each region. In particular, the casewhere change of integrated luminance with respect to a gray scale levelof one of subframes is constant in every region is described.

FIG. 8A shows an example of a method for distributing the totalintegrated luminance in one frame to 1SF and 2SF. A table under thegraph shows a feature of each subframe briefly. Each feature of 2SF in aregion 1, a region 2, and a region 3 is that change of integratedluminance with respect to a gray scale level is constant. Each featureof 1SF in the region 1, the region 2, and the region 3 is that theluminance depends on the total luminance and the luminance of the othersubframe.

FIG. 8B shows an example of a method for distributing the totalintegrated luminance in one frame to 1SF and 2SF. A table under thegraph shows a feature of each subframe briefly. Each feature of 2SF in aregion 1 and a region 2 and 1SF in a region 3 is that change ofintegrated luminance with respect to a gray scale is constant. Eachfeature of 1SF in the region 1 and the region 2 and 2SF in the region 3is that the luminance depends on the total luminance and the luminanceof the other subframe.

FIG. 8C shows an example of a method for distributing the totalintegrated luminance in one frame to 1SF and 2SF. A table under thegraph shows a feature of each subframe briefly. Each feature of 2SF in aregion 1 and a region 3 and 1SF in a region 2 is that change ofintegrated luminance with respect to a gray scale is constant. Eachfeature of 1SF in the region 1 and the region 3 and 2SF in the region 2is that the luminance depends on the total luminance and the luminanceof the other subframe.

FIG. 8D shows an example of a method for distributing the totalintegrated luminance in one frame to 1SF and 2SF. A table under thegraph shows a feature of each subframe briefly. Each feature of 1SF in aregion 1 and 2SF in a region 2 and a region 3 is that change ofintegrated luminance with respect to a gray scale is constant. Eachfeature of 2SF in the region 1 and 1SF in the region 2 and the region 3is that the luminance depends on the total luminance and the luminanceof the other subframe.

Note that a value of the slope may be positive, 0, or negative.Differences among these are not described in detail in FIGS. 8A to 8D,and a combination of these can be applied to every region. When theslope is positive or negative and luminance difference between 1SF and2SF is increased, motion blur can be efficiently reduced. When the slopeis positive or negative and luminance difference between 1SF and 2SF isreduced, flickers in displaying an image are reduced. Alternatively,when the slope is 0, image processing and applied voltage aresimplified, and a load of a peripheral driver circuit is reduced.Further, a phenomenon in which an unnatural contour is generated can bereduced. Moreover, since the maximum luminance in 1SF and 2SF can belowered, power consumption can be reduced.

As described above, luminance at the boundary of regions can be in anyof the following states: changing discontinuously toward higherluminance; being continuous; or changing discontinuously toward lowerluminance, as compared with that in an adjacent region on the lower grayscale level side. Differences among these are not described in detail inFIGS. 8A to 8D, and a combination of these can be applied to everyregion boundary. When the luminance changes discontinuously at theboundary of regions, and thus the luminance difference between 1SF and2SF is increased, motion blur can be efficiently reduced. When theluminance changes discontinuously at the boundary of regions, and thusthe luminance difference between 1SF and 2SF is reduced, flickers indisplaying an image are reduced. Alternatively, when the luminance iscontinuous at the boundary of regions, image processing and appliedvoltage are simplified, and a load of a peripheral driver circuit isreduced. Further, a phenomenon in which an unnatural contour isgenerated can be reduced. Moreover, since the maximum luminance in 1SFand 2SF can be lowered, power consumption can be reduced.

In the modes shown in FIGS. 8A to 8D, 1SF and 2SF can be exchanged, andeven when the characteristics of 1SF and 2SF are exchanged, a similareffect can be obtained. Although the luminance in 1SF is higher than theluminance in 2SF, the invention is not limited to this. The luminance in1SF may be lower than the luminance in 2SF. Note that when the totalluminance is nonlinear, the luminance in 2SF is preferably lower thanthe luminance in 1SF because the gray scale can be controlled moreeasily. A magnitude relation of luminance between 1SF and 2SF may beexchanged. Further, the magnitude relation of luminance between 1SF and2SF may be exchanged separately in each region. A region or regions inwhich the magnitude relation of luminance is exchanged may be only theregion 1, only the region 2, only the region 3, the region 1 and theregion 2, the region 2 and the region 3, the region 3 and the region 1,or the region 1, the region 2 and the region 3, for example.

Next, another mode of this embodiment mode is described with referenceto FIGS. 9A to 9F. FIGS. 9A to 9F each show an example of a method fordistributing the total integrated luminance in one frame to 1SF and 2SFin the case where gray scales which can be displayed are divided into aplurality of regions, for example, three regions and each subframe canhave different characteristics in each region. In particular, the casewhere change of integrated luminance with respect to a gray scale levelof one of subframes is constant in two regions out of the three regions,and an integrated luminance ratio between 1SF and 2SF is equal in eachgray scale in the other region is described.

FIG. 9A shows an example of a method for distributing the totalintegrated luminance in one frame to 1SF and 2SF. A table under thegraph shows a feature of each subframe briefly. A feature of 2SF in aregion 1 and a region 2 is that change of integrated luminance withrespect to a gray scale is constant. A feature of 1SF in the region 1and the region 2 is that the luminance depends on the total luminanceand the luminance of the other subframe. A feature of 1SF in a region 3and 2SF in the region 3 is that an integrated luminance ratio between1SF and 2SF is equal in each gray scale. A value of the ratio in thiscase (the level of the smaller luminance with respect to the largerluminance) is preferably greater than 0.5 and less than 1. Accordingly,motion blur can be efficiently reduced.

FIG. 9B shows an example of a method for distributing the totalintegrated luminance in one frame to 1SF and 2SF. A table under thegraph shows a feature of each subframe briefly. A feature of 2SF in aregion 1 and 1SF in a region 2 is that change of integrated luminancewith respect to a gray scale is constant. A feature of 1SF in the region1 and 2SF the region 2 is that the luminance depends on the totalluminance and the luminance of the other subframe. A feature of 1SF in aregion 3 and 2SF in the region 3 is that an integrated luminance ratiobetween 1SF and 2SF is equal in each gray scale. A value of the ratio inthis case (the level of the smaller luminance with respect to the largerluminance) is preferably greater than 0.5 and less than 1. Accordingly,motion blur can be efficiently reduced.

FIG. 9C shows an example of a method for distributing the totalintegrated luminance in one frame to 1SF and 2SF A table under the graphshows a feature of each subframe briefly. A feature of 2SF in a region 1and a region 3 is that change of integrated luminance with respect to agray scale is constant. A feature of 1SF in the region 1 and the region3 is that the luminance depends on the total luminance and the luminanceof the other subframe. A feature of 1SF in a region 2 and 2SF in theregion 2 is that an integrated luminance ratio between 1SF and 2SF isequal in each gray scale. A value of the ratio in this case (the levelof the smaller luminance with respect to the larger luminance) ispreferably greater than 0.1 and less than 0.5. Accordingly, theluminance difference between 1SF and 2SF on the lower gray scale levelside can be increased, and motion blur can be efficiently reduced.

FIG. 9D shows an example of a method for distributing the totalintegrated luminance in one frame to 1SF and 2SF A table under the graphshows a feature of each subframe briefly. A feature of 2SF in a region 1and 1SF in a region 3 is that change of integrated luminance withrespect to a gray scale is constant. A feature of 1SF in the region 1and 2SF in the region 3 is that the luminance depends on the totalluminance and the luminance of the other subframe. A feature of 1SF in aregion 2 and 2SF in the region 2 is that an integrated luminance ratiobetween 1SF and 2SF is equal in each gray scale. A value of the ratio inthis case (the level of the smaller luminance with respect to the largerluminance) is preferably greater than 0.1 and less than 0.5.Accordingly, the luminance difference between 1SF and 2SF on the lowergray scale level side can be increased, and motion blur can beefficiently reduced.

FIG. 9E shows an example of a method for distributing the totalintegrated luminance in one frame to 1SF and 2SF. A table under thegraph shows a feature of each subframe briefly. A feature of 2SF in aregion 2 and a region 3 is that change of integrated luminance withrespect to a gray scale is constant. A feature of 1SF in the region 2and the region 3 is that the luminance depends on the total luminanceand the luminance of the other subframe. A feature of 1SF in a region 1and 2SF in the region 1 is that an integrated luminance ratio between1SF and 2SF is equal in each gray scale. A value of the ratio in thiscase (the level of the smaller luminance with respect to the largerluminance) is preferably greater than 0.1 and less than 0.5.Accordingly, the luminance difference between 1SF and 2SF on the lowergray scale level side can be increased, and motion blur can beefficiently reduced.

FIG. 9F shows an example of a method for distributing the totalintegrated luminance in one frame to 1SF and 2SF. A table under thegraph shows a feature of each subframe briefly. A feature of 2SF in aregion 2 and 1SF in a region 3 is that change of integrated luminancewith respect to a gray scale is constant. A feature of 1SF in the region2 and 2SF in the region 3 is that the luminance depends on the totalluminance and the luminance of the other subframe. A feature of 1SF in aregion 1 and 2SF in the region 1 is that an integrated luminance ratiobetween 1SF and 2SF is equal in each gray scale. A value of the ratio inthis case (the level of the smaller luminance with respect to the largerluminance) is preferably greater than 0.1 and less than 0.5.Accordingly, the luminance difference between 1SF and 2SF on the lowergray scale level side can be increased, and motion blur can beefficiently reduced.

Note that a value of the slope may be positive, 0, or negative.Differences among these are not described in detail in FIGS. 9A to 9F,and a combination of these can be applied to every region. When theslope is positive or negative and luminance difference between 1SF and2SF is increased, motion blur can be efficiently reduced. When the slopeis positive or negative and luminance difference between 1SF and 2SF isdecreased, flickers in displaying an image are reduced. Alternatively,when the slope is 0, image processing and applied voltage aresimplified, and a load of a peripheral driver circuit is reduced.Further, a phenomenon in which an unnatural contour is generated can bereduced. Moreover, since the maximum luminance in 1SF and 2SF can belowered, power consumption can be reduced.

As described above, luminance at the boundary of regions can be in anyof the following states: changing discontinuously toward higherluminance; being continuous; or changing discontinuously toward lowerluminance, as compared with that in an adjacent region on the lower grayscale level side. Differences among these are not described in detail inFIGS. 9A to 9F, and a combination of these can be applied to everyregion boundary. When the luminance changes discontinuously at theboundary of regions and thus the luminance difference between 1SF and2SF is increased, motion blur can be efficiently reduced. When theluminance changes discontinuously at the boundary of regions and thusthe luminance difference between 1SF and 2SF is decreased, flickers indisplaying an image are reduced. Alternatively, when the luminance iscontinuous at the boundary of regions, image processing and appliedvoltage are simplified, and a load of a peripheral driver circuit isreduced. Further, a phenomenon in which an unnatural contour isgenerated can be reduced. Moreover, since the maximum luminance in 1SFand 2SF can be lowered, power consumption can be reduced.

In the modes shown in FIGS. 9A to 9F, 1SF and 2SF can be exchanged, andeven when the characteristics of 1SF and 2SF are exchanged, a similareffect can be obtained. Although the luminance in 1SF is higher than theluminance in 2SF, the invention is not limited to this. The luminance in1SF may be lower than the luminance in 2SF. Note that when the totalluminance is nonlinear, the luminance in 2SF be lower than the luminancein 1SF because the gray scale can be controlled more easily. Further, amagnitude relation of luminance between 1SF and 2SF may be exchangedseparately in each region. A region or regions in which the magnituderelation of luminance is exchanged may be only the region 1, only theregion 2, only the region 3, the region 1 and the region 2, the region 2and the region 3, the region 3 and the region 1, or the region 1 and theregion 2 and the region 3, for example.

Next, another mode of this embodiment mode is described with referenceto FIGS. 10A to 10D. FIGS. 10A to 10D each show an example of a methodfor distributing the total integrated luminance in one frame to 1SF and2SF in the case where gray scales which can be displayed are dividedinto a plurality of regions, for example, three regions and eachsubframe can have different characteristics in each region. Inparticular, the case where an integrated luminance ratio between 1SF and2SF is equal in each gray scale in two regions out of the three regions,and change of integrated luminance with respect to a gray scale of oneof subframes is constant in the other region is described. Moreover, thecase where an integrated luminance ratio between 1SF and 2SF is equal ineach gray scale in every region is also described.

FIG. 10A shows an example of a method for distributing the totalintegrated luminance in one frame to 1SF and 2SF A table under the graphshows a feature of each subframe briefly. A feature of 2SF in a region 1is that change of integrated luminance with respect to a gray scale isconstant. A feature of 1SF in the region 1 is that the luminance dependson the total luminance and the luminance of the other subframe. Afeature of 1SF in a region 2 and a region 3 and 2SF in the region 2 andthe region 3 is that an integrated luminance ratio between 1SF and 2SFis equal in each gray scale. A value of the ratio in the region 2 (thelevel of the smaller luminance with respect to the larger luminance) ispreferably greater than 0.1 and less than 0.5. Accordingly, theluminance difference between 1SF and 2SF on the lower gray scale levelside can be increased, and motion blur can be efficiently reduced.Further, a value of the ratio in the region 3 is preferably greater than0.5 and less than 1. Accordingly, motion blur can be efficientlyreduced.

FIG. 10B shows an example of a method for distributing the totalintegrated luminance in one frame to 1SF and 2SF. A table under thegraph shows a feature of each subframe briefly. A feature of 2SF in aregion 2 is that change of integrated luminance with respect to a grayscale is constant. A feature of 1SF in the region 2 is that theluminance depends on the total luminance and the luminance of the othersubframe. A feature of 1SF in a region 1 and a region 3 and 2SF in theregion 1 and the region 3 is that an integrated luminance ratio between1SF and 2SF is equal in each gray scale. A value of the ratio in theregion 1 (the level of the smaller luminance with respect to the largerluminance) is preferably greater than 0.1 and less than 0.5.Accordingly, the luminance difference between 1SF and 2SF on the lowergray scale level side can be increased, and motion blur can beefficiently reduced. Further, a value of the ratio in the region 3 ispreferably greater than 0.5 and less than 1. Accordingly, motion blurcan be efficiently reduced.

FIG. 10C shows an example of a method for distributing the totalintegrated luminance in one frame to 1SF and 2SF. A table under thegraph shows a feature of each subframe briefly. A feature of 2SF in aregion 3 is that change of integrated luminance with respect to a grayscale is constant. A feature of 1SF in the region 3 is that theluminance depends on the total luminance and the luminance of the othersubframe. A feature of 1SF in a region 1 and a region 2 and 2SF in theregion 1 and the region 2 is that an integrated luminance ratio between1SF and 2SF is equal in each gray scale. A value of the ratio in theregion 1 and the region 2 (the level of the smaller luminance withrespect to the larger luminance) is preferably greater than 0.1 and lessthan 0.5. Accordingly, the luminance difference between 1SF and 2SF onthe lower gray scale level side can be increased, and motion blur can beefficiently reduced.

FIG. 10D shows an example of a method for distributing the totalintegrated luminance in one frame to 1SF and 2SF. A table under thegraph shows a feature of each subframe briefly. A feature of 1SF in aregion 1, a region 2, and a region 3, and 2SF in the region 1, theregion 2, and the region 3 is that an integrated luminance ratio between1SF and 2SF is equal in each gray scale. A value of the ratio in theregion 1 and the region 2 (the level of the smaller luminance withrespect to the larger luminance) is preferably greater than 0.1 and lessthan 0.5. Accordingly, the luminance difference between 1SF and 2SF onthe lower gray scale level side can be increased, and motion blur can beefficiently reduced. A value of the ratio in the region 3 is preferablygreater than 0.5 and less than 1. Accordingly, motion blur can beefficiently reduced.

Note that a value of the slope may be positive, 0, or negative.Differences among these are not described in detail in FIGS. 10A to 10D,and a combination of these can be applied to every region. When theslope is positive or negative and luminance difference between 1SF and2SF is increased, motion blur can be efficiently reduced. When the slopeis positive or negative and luminance difference between 1SF and 2SF isreduced, flickers in displaying an image are reduced. Alternatively,when the slope is 0, image processing and applied voltage aresimplified, and a load of a peripheral driver circuit is reduced.Further, a phenomenon in which an unnatural contour is generated can bereduced. Moreover, since the maximum luminance in 1SF and 2SF can belowered, power consumption can be reduced.

As described above, luminance at the boundary of regions can be in anyof the following states: changing discontinuously toward higherluminance; being continuous; or changing discontinuously toward lowerluminance, as compared with that in an adjacent region on the lower grayscale level side. Differences among these are not described in detail inFIGS. 10A to 10D, and a combination of these can be applied to everyregion boundary. When the luminance changes discontinuously at theboundary of regions and thus the luminance difference between 1SF and2SF is increased, motion blur can be efficiently reduced. When theluminance changes discontinuously at the boundary of regions and thusthe luminance difference between 1SF and 2SF is reduced, flickers indisplaying an image are reduced. Alternatively, when the luminance iscontinuous at the boundary of regions, image processing and appliedvoltage are simplified, and a load of a peripheral driver circuit isreduced. Further, a phenomenon in which an unnatural contour isgenerated can be reduced. Moreover, since the maximum luminance in 1SFand 2SF can be lowered, power consumption can be reduced.

In the modes shown in FIGS. 10A to 10D, 1SF and 2SF can be exchanged,and even when the characteristics of 1SF and 2SF are exchanged, asimilar effect can be obtained. Although the luminance in 1SF is higherthan the luminance in 2SF, the invention is not limited to this. Theluminance in 1SF may be lower than the luminance in 2SF. Note that whenthe total luminance is nonlinear, the luminance in 2SF is preferablylower than the luminance in 1SF because the gray scale can be controlledmore easily. Further, a magnitude relation of luminance between 1SF and2SF may be exchanged separately in each region. A region or regions inwhich the magnitude relation of luminance is exchanged may be only theregion 1, only the region 2, only the region 3, the region 1 and theregion 2, the region 2 and the region 3, the region 3 and the region 1,or the region 1 and the region 2 and the region 3, for example.

Next, another mode of this embodiment mode is described with referenceto FIGS. 11A and 11B. FIGS. 11A and 11B each show an example of the casewhere the division number of gray scale levels to be displayed is 4 ormore. The division number of regions may be any number as long as pluralkinds of gray scales are included in each region. FIGS. 11A and 11Billustrate characteristic examples.

FIG. 11A shows an example of a method for distributing the totalintegrated luminance in one frame to 1SF and 2SF. Features of the methodshown in FIG. 11A are as follows: an image to be displayed in 2SF isused as a dark image, the number of levels of luminance of the image tobe displayed in 2SF is limited to be several, the luminance is graduallyincreased as a gray scale level becomes higher, and a gray scale isinterpolated by using a bright image in each region. Accordingly, itbecomes easy to form image data for displaying an image to be displayedin 2SF, and a load of a peripheral driver circuit is reduced. Moreover,since the number of levels of luminance to be expressed in 2SF isreduced when overdriving is combined, an overdrive circuit can besimplified. Note that the number of levels of luminance to be expressedin 2SF is preferably approximately from 4 to 16. In addition, thedivision number of gray scales which can be displayed is preferablyequal to the number of levels of luminance to be expressed in SF.

FIG. 11B shows an example of a method for distributing the totalintegrated luminance in one frame to 1SF and 2SF. Features of the methodshown in FIG. 11B are as follows: an image to be displayed in 1SF isused as a bright image, the number of levels of luminance of the imageto be displayed in 1SF is limited to be several, the luminance isgradually increased as a gray scale level becomes higher, a gray scaleis interpolated by using a dark image in each region, and the luminanceof the dark image is made close to 0 at the boundary of regions.Accordingly, it becomes easy to form image data for displaying an imageto be displayed in 1SF, and a load of a peripheral driver circuit isreduced. Further, since the number of levels of luminance to beexpressed in 1SF is reduced when overdriving is combined, an overdrivecircuit can be simplified. Furthermore, since average luminance of thedark image can be drastically reduced, motion blur can be significantlyreduced. Note that the number of levels of luminance to be expressed in1SF is preferably approximately from 16 to 64. In addition, the divisionnumber of gray scales which can be displayed is preferably equal to thenumber of kinds of luminance to be expressed in SF. Accordingly, astructure of a D/A converter can be simplified, for example. That is, adigital signal is treated as it is in one of the subframe periods, andthe amplitude of an analog signal is reduced (the number of kinds ofdiscrete values is reduced) in the other subframe period; thus, powerconsumption can be reduced, and the circuit size can be reduced. Notethat when an analog signal is employed in both the subframe periods, theamplitude of both of the analog signals is reduced; thus, powerconsumption can be reduced, and the circuit size can be reduced.

In the modes shown in FIGS. 11A and 11B, 1SF and 2SF can be exchanged,and even when the characteristics of 1SF and 2SF are exchanged, asimilar effect can be obtained. Although the luminance in 1SF is higherthan the luminance in 2SF, the invention is not limited to this. Theluminance in 1SF may be lower than the luminance in 2SF. Note that whenthe total luminance is nonlinear, the luminance in 2SF is preferablylower than the luminance in 1SF because the gray scale can be controlledmore easily. Further, a magnitude relation of luminance between 1SF and2SF may be exchanged separately in each region. A region or regions inwhich the magnitude relation of luminance is exchanged may be only theregion 1, only the region 2, only the region 3, the region 1 and theregion 2, the region 2 and the region 3, the region 3 and the region 1,or the region 1 and the region 2 and the region 3, for example. This canbe similarly applied to the region 4 and subsequent regions.

Next, another mode of this embodiment mode is described with referenceto FIGS. 12A to 12D. FIGS. 12A to 12D each show an example in which oneframe is divided into three subframes. The number of subframes is notlimited, but when it is 3, a particularly beneficial effect can beobtained. In this embodiment mode, a subframe period that is the firstperiod in one frame period is referred to as 1SF, a subframe period thatis the second period is referred to as 2SF, and a subframe period thatis the third period is referred to as 3SF.

In graphs of FIGS. 12A and 12B, the horizontal axis represents time, andthe vertical solid lines represent boundaries of frames. The verticaldashed lines represent boundaries of subframes. The vertical axisrepresents luminance. That is, each of FIGS. 12A and 12B shows change ofluminance of a pixel with respect to time over five frames.

The degree of gray scale in each frame is represented below thehorizontal axis. That is, each of FIGS. 12A and 12B shows change of theluminance of a pixel with respect to time in the case where a minimumgray scale is displayed first, and then, halftone on the lower grayscale level side, halftone of the intermediate degree, halftone on thehigher gray scale level side, and a maximum gray scale are displayed inthis order.

A feature of the methods shown in FIGS. 12A and 12B is that a gray scaleis expressed by changing the luminance in 1SF and 2SF and the luminancein 3SF is 0 or very low, so that a pseudo impulsive driving method canbe performed. FIG. 12A shows the case where a bright image is displayedin 2SF and a dark image is displayed in 1SF. FIG. 12B shows the casewhere a bright image is displayed in 1SF and a dark image is displayedin 2SF.

Note that since an effect of improving motion blur can be obtained bymaking the luminance in 3SF be 0 or very low, L_(max1) and L_(max2)which are the maximum luminance of 1SF and the maximum luminance of 2SFrespectively, are not particularly limited. However, similarly toEmbodiment Mode 3, when a dark image is inserted in 1SF, L_(max1) ispreferably in the range represented as follows:(1/2)L_(max2)<L_(max1)<(9/10)L_(max2). Further, when a dark image isinserted in 2SF, L_(max2) is preferably in the range represented asfollows: (1/2)L_(max1)<L_(max2)<(9/10)L_(max1).

FIG. 12C shows an example of a method for distributing the totalintegrated luminance in one frame to 1SF, 2SF, and 3SF. A table underthe graph shows a feature of each subframe briefly. A feature of 2SF ina region 1 and 1SF in a region 2 is that change of integrated luminancewith respect to a gray scale is constant. A feature of 1SF in the region1 and 2SF in the region 2 is that the luminance depends on the totalluminance and the luminance of the other subframe. The luminance of 3SFin the region 1 and the region 2 may be kept constant at 0. Accordingly,motion blur can be effectively reduced in all the regions.

FIG. 12D shows an example of a method for distributing the totalintegrated luminance in one frame to 1SF, 2SF, and 3SF. A table underthe graph shows a feature of each subframe briefly. A feature of 2SF ina region 1 and 1SF in a region 2 is that change of integrated luminancewith respect to a gray scale is constant. A feature of 1SF in the region1 and 2SF in the region 2 is that the luminance depends on the totalluminance and the luminance of the other subframe. Further, the slope ofluminance of 3SF in the region 1 and the region 2 may be kept constantat a small value. When the maximum luminance of 3SF is denoted byL_(max3), L_(max3) is preferably less than or equal to 1/10 of themaximum luminance of 1SF and the maximum luminance of 2SF. Accordingly,motion blur can be effectively reduced in all the gray scale regions.

Note that a value of the slope may be positive, 0, or negative.Differences among these are not described in detail in FIGS. 12A to 12D,and a combination of these can be applied to every region. When theslope is positive or negative and luminance difference between 1SF and2SF is increased, motion blur can be efficiently reduced. When the slopeis positive or negative and luminance difference between 1SF and 2SF isdecreased, flickers in displaying an image are reduced. Alternatively,when the slope is 0, image processing and applied voltage aresimplified, and a load of a peripheral driver circuit is reduced.Further, a phenomenon in which an unnatural contour is generated can bereduced. Moreover, since the maximum luminance in 1SF and 2SF can belowered, power consumption can be reduced.

As described above, luminance at the boundary of regions can be in anyof the following states: changing discontinuously toward higherluminance; being continuous; or changing discontinuously toward lowerluminance, as compared with that in an adjacent region on the lower grayscale level side. Differences among these are not described in detail inFIGS. 12A to 12D, and a combination of these can be applied to everyregion boundary. When the luminance changes discontinuously at theboundary of regions and thus the luminance difference between 1SF and2SF is increased, motion blur can be efficiently reduced. When theluminance changes discontinuously at the boundary of regions and thusthe luminance difference between 1SF and 2SF is reduced, flickers indisplaying an image are reduced. Alternatively, when the luminance iscontinuous at the boundary of regions, image processing and appliedvoltage are simplified, and a load of a peripheral driver circuit isreduced. Further, a phenomenon in which an unnatural contour isgenerated can be reduced. Moreover, since the maximum luminance in 1SFand 2SF can be lowered, power consumption can be reduced.

In the modes shown in FIGS. 12A to 12D, 1SF, 2SF and 3SF can beexchanged, and even when the characteristics of 1SF, 2SF and 3SF areexchanged, a similar effect can be obtained. Although the luminance in1SF is higher than the luminance in 2SF, the invention is not limited tothis. The luminance in 1SF may be lower than the luminance in 2SF. Notethat when the total luminance is nonlinear, the luminance in 2SF ispreferably lower than the luminance in 1SF because the gray scale can becontrolled more easily. A magnitude relation of luminance between 1SFand 2SF may be exchanged. Further, a region or regions in which themagnitude relation of luminance between 1SF and 2SF is exchanged may beonly the region 1, only the region 2, or the region 1 and the region 2.

All the modes described in this embodiment mode may be performed incombination with overdriving. Accordingly, response speed of a liquidcrystal display element can be increased, and quality of moving imagescan be improved.

All the modes described in this embodiment mode may be performed as aliquid crystal display device in combination with a scanning backlight.Accordingly, average luminance of a backlight can be reduced, so thatpower consumption can be reduced.

All the modes described in this embodiment mode may be performed incombination with high frequency driving. Accordingly, quality of movingimages can further be improved.

All the modes described in this embodiment mode may be performed incombination with a driving method in which objective voltage is appliedto a display element by operating a potential of a common line.Accordingly, the frequency of writing a video signal to a pixel isdecreased, so that power which is consumed when the video signal iswritten to the pixel can be reduced.

All the modes described in this embodiment mode may be performed incombination with a display element driven by a current, such as anorganic EL element. Accordingly, video signal current can be increased,and writing time can be reduced.

All the modes described in this embodiment mode may be performed incombination with interlace scanning. Accordingly, operating frequency ofa peripheral driver circuit can be reduced, and power consumption can bereduced. This is particularly effective in the case of a dark image withmany pixels in a non-lighting state or in the case of a bright imagewith many pixels emitting light with the maximum luminance. This isbecause reduction in resolution due to the interlace scanning is smallin an image with less change of gray scale level.

All the modes described in this embodiment mode may be performed incombination with a D/A converter circuit which can change a referencepotential. Accordingly, efficiency of the D/A converter circuit can beimproved. It is particularly effective that the reference potential canbe changed between a subframe for displaying a bright image and asubframe for displaying a dark image. This is because an average valueof a potential needed for a video signal is different in displaying abright image and a dark image.

Note that although this embodiment mode is described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, even more drawings can be formed by combining each part withanother part in the above-described drawings.

Similarly, the contents (or part of the contents) described in eachdrawing of this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, even more drawings can beformed by combining each part with part of another embodiment mode inthe drawings of this embodiment mode.

This embodiment mode shows an example of an embodied case of thecontents (or part of the contents) described in another embodiment mode,an example of slight transformation thereof, an example of partialmodification thereof, an example of improvement thereof, an example ofdetailed description thereof, an application example thereof, an exampleof related part thereof, or the like. Therefore, the contents describedin another embodiment mode can be freely applied to, combined with, orreplaced with this embodiment mode.

Embodiment Mode 5

In this embodiment mode, a silicon on insulator (SOI) substrate isdescribed. Specifically, a method of forming an SOI substrate which isformed by transposition from a single-crystalline semiconductorsubstrate to a different substrate (hereinafter also referred to as abase substrate).

Note that in a transistor formed using an SOI substrate, parasiticcapacitance is small and a short channel effect is suppressed comparedwith the case of a transistor formed using a normal single-crystallinesemiconductor substrate. Further, the transistor formed using the SOIsubstrate has high mobility, low driving voltage, and littledeterioration over time and little variation in characteristics comparedwith the case of a normal thin film transistor (including a thin filmtransistor using amorphous silicon or polycrystalline silicon). When theSOI substrate with which a transistor having such characteristics can beformed is applied to a variety of devices, various problems that aconventional device has can be solved.

For example, when a device whose performance can be improved by making atransistor minuter (e.g., a central processing unit (CPU) or asemiconductor memory) is formed using the SOI substrate, a short channeleffect of the transistor can be suppressed; thus, the transistor can befurther made minuter, and the performance can be improved.

In addition, when a device in which a thin film transistor is suitableto be used (e.g., a display device) is formed using the SOI substrate,various characteristics of the display device can be improved asfollows, for example. Power consumption can be reduced by improvement inaperture ratio of pixels (the size of the transistor can be reduced).Power consumption can be reduced by reduction in driving voltage.Reliability can be improved (deterioration over time can be reduced).Quality of display images can be improved (variation in characteristicsof the transistor can be reduced).

Specifically, characteristics of a liquid crystal display device formedusing the SOI substrate can be significantly improved. For example, whena transistor in a pixel is formed using the SOI substrate, an apertureratio of the liquid crystal display device can be increased; thus, powerconsumption can be reduced. When a peripheral driver circuit such as agate driver or a source driver is formed using the SOI substrate,driving voltage of the circuit can be reduced; thus, power consumptioncan be reduced. When an image data processing circuit, a timinggeneration circuit, or the like is formed using the SOI substrate,driving voltage of the circuit can be reduced; thus, power consumptioncan be reduced. The use of the SOI substrate is more effective when theliquid crystal display device can perform overdriving. This is becauseimage data is frequently processed by overdriving, and thus an effect ofreducing power consumption due to reduction in driving voltage is moresignificant. Further, in the case of a circuit using a memory, such as alookup table, an effect of reducing power consumption due to reductionin driving voltage is significant. Similarly, when a device has astructure capable of performing driving (double-frame rate driving) inwhich a frame rate of image data input to the liquid crystal displaydevice is converted into a higher frame rate to perform display, the useof the SOI substrate is quite effective. This is because while drivingfrequency of a pixel circuit and a peripheral driver circuit becomessignificantly high because of double-frame rate driving, driving voltagecan be reduced when the circuit is formed using the SOI substrate; thus,power consumption can be drastically reduced.

Next, a method of forming an SOI substrate is described.

An SOI substrate 2400 has a structure where a plurality of stacks oflayers in each of which an insulating layer 2420 and asingle-crystalline semiconductor layer 2430 (hereinafter also referredto as an SOI layer) are stacked in this order are provided over asurface of a base substrate 2410 (see FIGS. 24F and 24G). The SOI layer2430 is provided over the base substrate 2410 with the insulating layer2420 interposed therebetween and forms a so-called SOI structure. Notethat a plurality of SOI layers may be provided over one base substrate2410 and form one SOI substrate 2400. FIGS. 24F and 24G show exampleswhere two SOI layers 2430 are provided over one base substrate 2410.

The SOI layer 2430 is a single-crystalline semiconductor, andsingle-crystalline silicon can be typically used. Alternatively, acrystalline semiconductor layer of silicon, germanium, a compoundsemiconductor such as gallium arsenide or indium phosphide, or the like,which can be separated from a single-crystalline semiconductor substrateor a polycrystalline semiconductor substrate by a hydrogen ionimplantation separation method, can be used.

Note that the size of the SOI layer 2430 which forms the SOI substratemay be approximately the same as a desired panel size. A panel sizerefers to the size of the sum of a display portion of a display paneland a peripheral frame portion (a non-display portion). Moreover, a sizerefers to an area.

The panel size may be selected as appropriate depending on applications,and for example, a medium or small panel size of a diagonal of less than10 inches can be employed. When a medium or small panel is employed fora mobile phone, known sizes (screen sizes) of a display portion arediagonal of 2.2 inches (56 mm), 2.4 inches (61 mm), and 2.6 inches (66mm), for example. When a mobile phone has the above panel size, thepanel size may be determined in consideration of the size of a frameportion (the screen frame size) around a display portion in addition tothe screen size.

Although a shape of the SOI layer 2430 is not particularly limited, itis preferable to employ a rectangular shape (including a square) becauseprocessing becomes easy and the SOI layer 2430 can be attached to thebase substrate 2410 with a high integration degree. When the SOI layer2430 is used for a panel of a display device, such a display, the SOIlayer 2430 preferably has an aspect ratio of 4:3. When the SOI layer2430 has approximately the same size as a desired panel, it is possibleto control a yield on a panel-to-panel basis in manufacturing a varietyof display devices by incorporating a display panel formed using acompleted SOI substrate. Further, damage to elements can be preventedwhen the panels are cut from each other. Accordingly, a yield can beimproved. Moreover, when the SOI layer 2430 has approximately the samesize as the desired panel, respective elements of panels can be formedusing one SOI layer, and thus variation in characteristics can besuppressed.

A substrate having an insulating surface or an insulating substrate isused for the base substrate 2410. Specifically, a variety of glasssubstrates used for electronic industries (e.g. aluminosilicate glass,aluminoborosilicate glass, or barium borosilicate glass), a quartzsubstrate, a ceramic substrate, or a sapphire substrate can be given asan example. It is preferable to use a glass substrate for the basesubstrate 2410, and for example, a large-sized mother glass substratecalled the sixth generation (1500 mm×1850 mm), the seventh generation(1870 mm×2200 mm), or the eighth generation (2200 mm×2400 mm) is used.When a large-sized mother glass is used for the base substrate 2410 andan SOI substrate is formed by applying the invention, increase in areaof the SOI substrate can be realized. Further, when each SOI layer has adesired panel size, the number of display panels which can be formedusing one base substrate can be increased. Accordingly, productivity ofend products (display devices) formed with the display panelincorporated therein can be improved.

The insulating layer 2420 is provided between the base substrate 2410and the SOI layer 2430. The insulating layer 2420 may have asingle-layer structure or a stacked-layer structure. A surface bonded tothe base substrate 2410 (hereinafter also referred to as a bondingsurface) is smooth and hydrophilic. FIG. 24F shows an example in which abonding layer 2422 is formed as the insulating layer 2420. A siliconoxide layer is suitable for the bonding layer 2422 which has a smoothsurface and can form a hydrophilic surface. In particular, a siliconoxide layer which is formed by a chemical vapor deposition method withthe use of organic silane is preferable. As the organic silane, acompound containing silicon such as tetraethoxysilane (TEOS:Si(OC₂H₅)₄),tetramethylsilane (TMS:Si(CH₃)₄), trimethylsilane ((CH₃)₃SIH),tetramethylcyclotetrasiloxane (TMCTS), octamethylcyclotetrasiloxane(OMCTS), hexamethyldisilazane (HMDS), triethoxysilane (SiH(OC₂H₅)₃), ortrisdimethylaminosilane (SiH(N(CH₃)₂)₃) can be used.

The bonding layer 2422 which has a smooth surface and forms ahydrophilic surface preferably has a thickness of 5 to 500 nm. When thebonding layer 2422 has a thickness within the above range, it ispossible to smooth roughness of a surface of a film is to be formed andensure smoothness of a developing surface of the bonding layer 2422.Further, a distortion between the bonding layer 2422 and a substratebonded thereto (in FIG. 24F, the base substrate 2410) can be relieved.Note that a silicon oxide layer which is similar to the bonding layer2422 may be provided in the base substrate 2410. In the SOI substrateaccording to the invention, when the SOI layer 2430 is bonded to thebase substrate 2410 which is a substrate having an insulating surface oran insulating substrate, a bonding layer which is preferably formed of asilicon oxide layer using organic silane is provided in one or both ofthe surfaces on which a bond is formed, and thus, a firm bond can beformed.

FIG. 24G shows an example in which the insulating layer 2420 has astacked-layer structure, and specifically, an example in which astacked-layer structure of the bonding layer 2422 and an insulatinglayer 2424 containing nitrogen is formed as the insulating layer 2420.The insulating layer 2424 containing nitrogen is provided between theSOI layer 2430 and the bonding layer 2422 so that the bonding layer 2422is formed in a surface bonded to the base substrate 2410. The insulatinglayer 2424 containing nitrogen is formed to have a single-layerstructure or a stacked-layer structure of a silicon nitride layer, asilicon nitride oxide layer, and/or a silicon oxynitride layer. Forexample, the insulating layer 2424 containing nitrogen can be formed bysequentially stacking a silicon oxynitride layer and a silicon nitrideoxide layer from the SOI layer 2430 side. The bonding layer 2422 isprovided in order to form a bond with the base substrate 2410. Theinsulating layer 2424 containing nitrogen is preferably provided inorder to prevent impurities such as mobile ions or moisture fromdiffusing into the SOI layer 2430 and contaminating the SOI layer 2430.

Note that a silicon oxynitride layer refers to a film that contains moreoxygen than nitrogen and contains oxygen, nitrogen, silicon, andhydrogen at concentrations ranging from 55 to 65 at. %, 1 to 20 at. %,25 to 35 at. %, and 0.1 to 10 at. %, respectively. Further, a siliconnitride oxide layer refers to a film that contains more nitrogen thanoxygen and contains oxygen, nitrogen, silicon, and hydrogen atconcentrations ranging from 15 to 30 at. %, 20 to 35 at. %, 25 to 35 at.%, and 15 to 25 at. %, respectively.

FIGS. 26D and 26E show examples in which an insulating layer 2450including a bonding layer is formed over the base substrate 2410. Theinsulating layer 2450 may have a single-layer structure or astacked-layer structure as long as a surface bonded to the SOI layer2430 is smooth and hydrophilic. Note that it is preferable to provide abarrier layer between the base substrate 2410 and the bonding layer inorder to prevent mobile ions such as alkali metal or alkaline earthmetal from diffusing from the glass substrate used as the base substrate2410.

FIGS. 26D and 26E show examples in which a stacked-layer structure of abarrier layer 2452 and a bonding layer 2454 is formed as the insulatinglayer 2450. As the bonding layer 2454, a silicon oxide layer which issimilar to the bonding layer 2422 may be provided. Further, the SOIlayer 2430 may be provided with a bonding layer as appropriate. FIG. 26Dshows an example in which the SOI layer 2430 is also provided with thebonding layer 2422. With such a structure, the bonding layers form abond when the SOI layer 2430 and the base substrate 2410 are bonded;thus, a firmer bond can be formed. The barrier layer 2452 is formed tohave a single-layer structure or a stacked-layer structure of a siliconoxide layer, a silicon nitride layer, a silicon oxynitride layer, and/ora silicon nitride oxide layer. Preferably, the barrier layer 2452 isformed using an insulating layer containing nitrogen.

FIG. 26E shows an example in which the base substrate 2410 is providedwith a bonding layer, and specifically, an example in which astacked-layer structure of the barrier layer 2452 and the bonding layer2454 is provided as the insulating layer 2450 over the base substrate2410. Further, the SOI layer 2430 is provided with a silicon oxide layer2426. When the SOI layer 2430 is bonded to the base substrate 2410, thesilicon oxide layer 2426 forms a bond with the bonding layer 2454. It ispreferable to form the silicon oxide layer 2426 by a thermal oxidationmethod. Alternatively, chemical oxide may be used for the silicon oxidelayer 2426. Chemical oxide can be formed by processing a surface of asemiconductor substrate with water containing ozone, for example.Chemical oxide is preferable because it is formed reflecting planarityof the surface of the semiconductor substrate.

Next, a method of forming an SOI substrate according to the invention isdescribed. First, an example of a method of forming the SOI substrateshown in FIG. 24F is described.

First, a semiconductor substrate 2401 is prepared (see FIGS. 24A and25A). As the semiconductor substrate 2401, a commercial semiconductorsubstrate such as a silicon substrate, a germanium substrate, or acompound semiconductor (e.g. gallium arsenide or indium phosphide)substrate can be used. Typical sizes of the commercial silicon substrateare 5 inches (125 mm), 6 inches (150 mm), 8 inches (200 mm), and 12inches (300 mm) in diameter, and most of the commercial siliconsubstrates are circular. Further, the film thickness can be selected upto approximately 1.5 mm as appropriate.

Next, ions 2404 accelerated by an electric field are implanted at apredetermined depth from a surface of the semiconductor substrate 2401,and an ion-doped layer 2403 is formed (see FIGS. 24A and 25A). The ions2404 are introduced in consideration of the thickness of an SOI layerwhich is transferred to a base substrate later. The SOI layer preferablyhas a thickness of 5 to 500 nm, and more preferably 10 to 200 nm.Acceleration voltage and the dose of the ions in ion introduction areselected as appropriate in consideration of the thickness of the SOIlayer to be transferred. As the ions 2404, ions of hydrogen, helium, orhalogen such as fluorine can be used. Note that as the ion 2404, it ispreferable to use an ion species including an atom or a plurality of thesame atoms formed by exciting a source gas selected from hydrogen,helium, and a halogen element with plasma. When a hydrogen ion isintroduced, it is preferable that H⁺, H₂ ⁺, and H₃ ⁺ ions be containedand the H₃ ⁺ ion be contained at a higher percentage because efficiencyof ion introduction can be improved and the time of ion introduction canbe reduced. Further, such a structure enables easy separation.

In order to form the ion-doped layer 2403 at a predetermined depth, theions 2404 need to be introduced at a high dose rate in some cases. Inthis case, the surface of the semiconductor substrate 2401 becomes roughdepending on conditions. Therefore, a silicon nitride layer, a siliconnitride oxide layer, or the like with a thickness of 50 to 200 nm may beprovided as a protective layer on the surface of the semiconductorsubstrate, in which the ions are introduced.

Next, the bonding layer 2422 is formed on the semiconductor substrate2401 (see FIGS. 24B and 25B). The bonding layer 2422 is formed on thesurface of the semiconductor substrate 2401, which forms a bond with thebase substrate. As the bonding layer 2422 formed here, a silicon oxidelayer formed by a chemical vapor deposition method with the use oforganic silane as a source gas as described above is preferablyemployed. It is also possible to use a silicon oxide layer formed by achemical vapor deposition method with the use of silane as a source gas.In film formation by a chemical vapor deposition method, temperatures atwhich degasification from the ion-doped layer 2403 formed in thesemiconductor substrate 2401 does not occur are applied. For example, afilm formation temperature of 350° C. or lower is applied. Note that aheat treatment temperature higher than the film formation temperature bya chemical vapor deposition method is applied to heat treatment forseparating the SOI layer from the semiconductor substrate such as asingle crystalline semiconductor substrate or a polycrystallinesemiconductor substrate.

Next, the semiconductor substrate 2401 can be processed into desiredsize and shape (see FIGS. 24C and 25C). Specifically, the semiconductorsubstrate 2401 can be processed into a desired panel size. FIG. 25Cshows an example in which the circular semiconductor substrate 2401 isdivided to form rectangular semiconductor substrates 2402. At this time,the bonding layer 2422 and the ion-doped layer 2403 are also divided.Thus, the ion-doped layer 2403 is formed at a predetermined depth, thebonding layer 2422 formed on the surface (the surface to which the basesubstrate is bonded), and the semiconductor substrates 2402 which has adesired panel size can be obtained.

The semiconductor substrate 2402 preferably has a panel size of avariety of display devices. The panel size may be selected asappropriate in accordance with an end product into which the panel isincorporated, or the like. For example, the panel size may be a diagonalof less than 10 inches, which is a panel size of a medium or smallpanel. For example, when the semiconductor substrate 2402 is applied toa mobile phone with a screen size of 2.4 inches in diagonal, the panelsize is determined in consideration of a screen frame size in additionto a screen size of 2.4 inches in diagonal. The shape of thesemiconductor substrate 2402 may also be determined as appropriatedepending on an application such as an end product. When thesemiconductor substrate 2402 is applied to a display device such as adisplay, the semiconductor substrate 2402 preferably has a rectangularshape with an aspect ratio of approximately 3:4. Further, thesemiconductor substrate 2402 preferably has a rectangular shape becauseprocessing in later manufacturing steps is easy and the semiconductorsubstrate 2402 can be efficiently obtained from the semiconductorsubstrate 2401. The semiconductor substrate 2401 can be cut with acutting device such as a dicer or a wire saw, a laser, plasma, anelectronic beam, or any other cutting means.

The order of steps up to formation of the bonding layer on the surfaceof the semiconductor substrate can be changed as appropriate. Thisembodiment mode shows an example in which the ion-doped layer is formedin the semiconductor substrate, the bonding layer is provided on thesurface of the semiconductor substrate, and then the semiconductorsubstrate is processed into a desired panel size. Alternatively, thefollowing order of steps may be used: after a semiconductor substrate isprocessed into a desired panel size, an ion-doped layer can be formed inthe semiconductor substrate having the desired panel size, and then, abonding layer can be formed on the surface of the semiconductorsubstrate having the desired panel size.

Next, the base substrate 2410 and the semiconductor substrate 2402 arebonded to each other. FIG. 24D shows an example in which the surface ofthe semiconductor substrate 2402 provided with the bonding layer 2422 isdisposed in contact with the base substrate 2410, and the base substrate2410 and the bonding layer 2422 are bonded to each other, so that thebase substrate 2410 and the semiconductor substrate 2402 are bonded toeach other. Note that it is preferable that the surface which forms abond (the bonding surface) be cleaned sufficiently. The bond is formedby the contact between the base substrate 2410 and the bonding layer2422. Van der Waals force acts on this bond, and the base substrate 2410and the semiconductor substrate 2402 are bonded to each other by beingpressed; thus, the firm bond due to hydrogen bonding can be formed.

The bonding surface may be activated in order to form a favorable bondbetween the base substrate 2410 and the bonding layer 2422. For example,at least one of the surfaces on which the bond is formed is irradiatedwith an atomic beam or an ion beam. When an atomic beam or an ion beamis utilized, an inert gas (e.g. argon) neutral atom beam or an inert gasion beam can be used. It is also possible to activate the bondingsurface by plasma irradiation or radical treatment. Such surfacetreatment facilitates formation of a bond between different materialseven at a temperature of 400° C. or lower

In addition, after the semiconductor substrate 2402 and the basesubstrate 2410 are bonded with the bonding layer 2422 interposedtherebetween, it is preferable to perform heat treatment or pressuretreatment. The heat treatment or the pressure treatment can improve abonding strength. A process temperature of the heat treatment ispreferably lower than or equal to the heat-resistant temperature of thebase substrate 2410. The pressure treatment is performed so thatpressure is applied in a direction perpendicular to the bonding surfacein consideration of pressure resistance of the base substrate 2410 andthe semiconductor substrate 2402.

Next, heat treatment is performed, and part of the semiconductorsubstrate 2402 is separated from the base substrate 2410 with theion-doped layer 2403 used as a cleavage plane (see FIG. 24E). A processtemperature of the heat treatment is preferably higher than or equal tothe film formation temperature of the bonding layer 2422 and lower thanor equal to the heat-resistant temperature of the base substrate 2410.For example, when heat treatment is performed at a temperature of 400 to600° C., the volume of a minute cavity formed in the ion-doped layer2403 is changed, and thus cleavage along the ion-doped layer 2403 ispossible. Since the bonding layer 2422 is bonded to the base substrate2410, the SOI layer 2430 which has the same crystallinity as thesemiconductor substrate 2402 is left over the base substrate 2410.

As described above, the SOI structure in which the SOI layer 2430 isprovided over the base substrate 2410 with the bonding layer 2422interposed therebetween is formed. Note that the SOI substrate can havea structure in which a plurality of SOI layers are provided over onebase substrate with a bonding layer interposed therebetween. Forexample, an ion-doped layer is formed, a bonding layer is formed on asurface, and the desired number of semiconductor substrates 2402 formedby being processed into sections each having a desired panel size areprepared. Then, after the desired number of semiconductor substrates2402 is bonded to the base substrate 2410 as shown in FIG. 27A,separation is performed at one time by heat treatment as shown in FIG.27B; thus, an SOI substrate can be formed. Note that instead ofperforming separation at one time by heat treatment, it is also possibleto repeat the steps of bonding and separation of one or somesemiconductor substrates 2402 to form an SOI substrate.

It is preferable to regularly arrange the semiconductor substrates 2402over the base substrate 2410 because such arrangement makes later stepseasy. For example, by using a control system such as a CCD camera or acomputer, the semiconductor substrates 2402 can be systematicallyarranged and bonded. Alternatively, a marker or the like may be formedover the base substrate 2410 or the semiconductor substrates 2402 toadjust the positions. Although FIGS. 27A and 27B show a structure inwhich adjacent SOI layers have some space therebetween, the SOI layersmay be arranged with as little space as possible.

Note that chemical mechanical polishing (CMP) may be performed on theSOI layers obtained by separation in order to planarize the surfaces.Alternatively, planarization may be performed by irradiating thesurfaces of the SOI layers with a laser beam instead of using a physicalpolishing method such as CMP. Laser beam irradiation is preferablyperformed in a nitrogen atmosphere containing oxygen at a concentrationof 10 ppm or less. This is because the surfaces of the SOI layers mightbe rough when laser beam irradiation is performed in an oxygenatmosphere. Further, CMP or the like may be performed to thin theobtained SOI layers.

Next, steps of providing a bonding layer on the base substrate side andforming an SOI layer as shown in FIG. 26D are described.

FIG. 26A shows a step of introducing the ions 2404 accelerated by anelectric field in the semiconductor substrate 2401 including the siliconoxide layer 2426 at a predetermined depth to form the ion-doped layer2403. The silicon oxide layer 2426 can be formed by a CVD method or asputtering method, preferably by a thermal oxidation method. As thesilicon oxide layer 2426, it is also possible to use chemical oxideformed by processing the surface of the semiconductor substrate withwater containing ozone, for example. The semiconductor substrate 2401can be similar to that in FIG. 24A. Further, introduction of ions ofhydrogen, helium, or halogen such as fluorine is performed in a similarmanner to that shown in FIG. 24A. When the silicon oxide layer 2426 isformed on the surface of the semiconductor substrate 2401, loss ofplanarity due to damage to the surface of the semiconductor substrate inthe ion introduction can be prevented.

FIG. 26B shows a step of disposing the surface of the semiconductorsubstrate 2402 provided with the silicon oxide layer 2426, in contactwith the base substrate 2410 provided with the barrier layer 2452 andthe bonding layer 2454, thereby forming a bond. The bond is formed bydisposing the silicon oxide layer 2426 over the semiconductor substrate2402 in contact with the bonding layer 2454 over the base substrate2410. The semiconductor substrate 2402 is obtained by processing thesemiconductor substrate 2401, in which the separation layer 2403 isformed and the silicon oxide layer 2426 is formed on the surface, intosections each having a desired panel size. The barrier layer 2452 may beformed using a silicon oxide layer, a silicon nitride layer, a siliconoxynitride layer, and/or a silicon nitride oxide layer to have asingle-layer structure or a stacked-layer structure by a CVD method or asputtering method. As the bonding layer 2454, a silicon oxide layerwhich is similar to the bonding layer 2422 may be formed.

Then, as shown in FIG. 26C, part of the semiconductor substrate 2402 isseparated. Heat treatment for the separation is performed in a similarmanner to that shown in FIG. 24E, and the part of the semiconductorsubstrate 2402 is separated from the base substrate 2410 with theion-doped layer 2403 used as a cleavage plane. After the separation, theSOI layer 2430 having the same crystallinity as the semiconductorsubstrate 2402 is left over the base substrate 2410; thus, the SOIsubstrate can be obtained. The SOI substrate can have a structure inwhich the SOI layer 2430 is provided over the base substrate 2410 withthe barrier layer 2452, the bonding layer 2454, and the silicon oxidelayer 2426 interposed therebetween. Note that after the separation, CMP,laser beam irradiation, or the like may be performed to planarize orthin the obtained SOI layer.

With the manufacturing method of the SOI substrate described in thisembodiment mode, the SOI layer 2430 having a bonding portion with a highbond strength can be obtained even when the base substrate 2410 has aheat-resistant temperature of 600° C. or lower. Further, a variety ofglass substrates used for electronic industry which are called analkali-free glass substrate, such as a substrate made of aluminosilicateglass, aluminoborosilicate glass, or barium borosilicate glass can beused as the base substrate 2410 because a process temperature of 600° C.or lower can be applied. It is needless to say that a ceramic substrate,a sapphire substrate, a quartz substrate, or the like can also be used.That is, a single crystalline semiconductor layer can be formed over asubstrate with a side more than 1 meter long. With the use of such alarge-sized substrate, a display device such as a liquid crystal displaydevice or a semiconductor integrated circuit can be manufactured.

The SOI substrate described in this embodiment mode has a structure inwhich panel-sized SOI layers are provided over a base substrate. Such astructure enables formation of desired display panels using one SOIlayer and can achieve improvement in yield. Further, desired displaypanels can be formed using one SOI layer, and thus variations inelements which form the display panels can be suppressed.

In addition, in the SOI substrate described in this embodiment mode, theyield can be controlled on a panel-to-panel basis even if a defectoccurs in a crystal of the SOI layer in transferring the SOI layer tothe base substrate. Moreover, even if different kinds of materials arebonded to each other, stress can be relieved because the SOI layers eachhaving a panel size are transferred to the base substrate; thus,improvement in yield can be realized.

In addition, the SOI substrate described in this embodiment mode canhave a large area by providing a plurality of SOI layers over a basesubstrate. Accordingly, a large number of display panels can bemanufactured by one series of manufacturing steps, and thus productivityof end products manufactured by incorporating the display panel can beimproved.

Note that although this embodiment mode is described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, even more drawings can be formed by combining each part withanother part in the above-described drawings.

Similarly, the contents (or part of the contents) described in eachdrawing of this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, even more drawings can beformed by combining each part with part of another embodiment mode inthe drawings of this embodiment mode.

This embodiment mode shows an example of an embodied case of thecontents (or part of the contents) described in another embodiment mode,an example of slight transformation thereof, an example of partialmodification thereof, an example of improvement thereof, an example ofdetailed description thereof, an application example thereof, an exampleof related part thereof, or the like. Therefore, the contents describedin another embodiment mode can be freely applied to, combined with, orreplaced with this embodiment mode.

Embodiment Mode 6

In this embodiment mode, a structure and a manufacturing method of atransistor are described.

FIGS. 28A to 28G show examples of structures and manufacturing methodsof transistors. FIG. 28A shows structure examples of transistors. FIGS.28B to 28G show examples of manufacturing methods of the transistors.

Note that the structure and the manufacturing method of the transistorsare not limited to those shown in FIGS. 28A to 28G, and variousstructures and manufacturing methods can be employed.

First, structure examples of transistors are described with reference toFIG. 28A. FIG. 28A is a cross-sectional view of a plurality oftransistors each having a different structure. Here, in FIG. 28A, theplurality of transistors each having a different structure arejuxtaposed, which is for describing structures of the transistors.Accordingly, the transistors are not needed to be actually juxtaposed asshown in FIG. 28A and can be separately formed as needed.

Next, characteristics of each layer forming the transistor aredescribed.

A substrate 110111 can be a glass substrate using barium borosilicateglass, aluminoborosilicate glass, or the like, a quartz substrate, aceramic substrate, a metal substrate containing stainless steel, or thelike. Further, a substrate formed of plastics typified by polyethyleneterephthalate (PET), polyethylene naphthalate (PEN), or polyethersulfone(PES), or a substrate formed of a flexible synthetic resin such asacrylic can also be used. By using a flexible substrate, a semiconductordevice capable of being bent can be formed. A flexible substrate has nostrict limitations on the area or the shape of the substrate.Accordingly, for example, when a substrate having a rectangular shape,each side of which is 1 meter or more, is used as the substrate 110111,productivity can be significantly improved. Such an advantage is highlyfavorable as compared with the case where a circular silicon substrateis used.

An insulating film 110112 functions as a base film and is provided toprevent alkali metal such as Na or alkaline earth metal from thesubstrate 110111 from adversely affecting characteristics of asemiconductor element. The insulating film 110112 can have asingle-layer structure or a stacked-layer structure of an insulatingfilm containing oxygen or nitrogen, such as silicon oxide (SiO_(x)),silicon nitride (SiN_(x)), silicon oxynitride (SiO_(x)N_(y)) (x>y), orsilicon nitride oxide (SiN_(x)O_(y)) (x>y). For example, when theinsulating film 110112 is provided to have a two-layer structure, it ispreferable that a silicon nitride oxide film be used as a firstinsulating film and a silicon oxynitride film be used as a secondinsulating film. As another example, when the insulating film 110112 isprovided to have a three-layer structure, it is preferable that asilicon oxynitride film be used as a first insulating film, a siliconnitride oxide film be used as a second insulating film, and a siliconoxynitride film be used as a third insulating film.

Semiconductor layers 110113, 110114, and 110115 can be formed using anamorphous semiconductor or a semi-amorphous semiconductor (SAS).Alternatively, a polycrystalline semiconductor layer may be used. SAS isa semiconductor having an intermediate structure between amorphous andcrystalline (including single crystal and polycrystalline) structuresand having a third state which is stable in free energy. Moreover, SASincludes a crystalline region with a short-range order and latticedistortion. A crystalline region of 0.5 to 20 nm can be observed atleast in part of a film. When silicon is contained as a main component,Raman spectrum shifts to a wave number side lower than 520 cm⁻¹. Thediffraction peaks of (111) and (220) which are thought to be contributedto a silicon crystalline lattice are observed by X-ray diffraction. SAScontains hydrogen or halogen of at least 1 atomic percent or more tocompensate dangling bonds. SAS is formed by glow discharge decomposition(plasma CVD) of a material gas. As the material gas, Si₂H₆, SiH₂Cl₂,SiHCl₃, SiCl₄, SiF₄, or the like as well as SiH₄ can be used.Alternatively, GeF₄ may be mixed. The material gas may be diluted withH₂, or H₂ and one or more kinds of rare gas elements selected from He,Ar, Kr, and Ne. A dilution ratio is in the range of 2 to 1000 times.Pressure is in the range of approximately 0.1 to 133 Pa, and a powersupply frequency is 1 to 120 MHz, preferably 13 to 60 MHz. A substrateheating temperature may be 300° C. or lower. A concentration ofimpurities in atmospheric components such as oxygen, nitrogen, andcarbon is preferably 1×10²⁰ cm⁻¹ or less as impurity elements in thefilm. In particular, an oxygen concentration is 5×10¹⁹/cm³ or less,preferably 1×10¹⁹/cm³ or less. Here, an amorphous semiconductor layer isformed using a material containing silicon (Si) as its main component(e.g., Si_(x)Ge_(1-x)) by a known method (such as a sputtering method,an LPCVD method, or a plasma CVD method). Then, the amorphoussemiconductor layer is crystallized by a known crystallization methodsuch as a laser crystallization method, a thermal crystallization methodusing RTA or an annealing furnace, or a thermal crystallization methodusing a metal element which promotes crystallization.

An insulating film 110116 can have a single-layer structure or astacked-layer structure of an insulating film containing oxygen ornitrogen, such as silicon oxide (SiO_(x)), silicon nitride (SiN_(x)),silicon oxynitride (SiO_(x)N_(y)) (x>y), or silicon nitride oxide(SiN_(x)O_(y)) (x>y).

A gate electrode 110117 can have a single-layer structure of aconductive film or a stacked-layer structure of two or three conductivefilms. As a material for the gate electrode 110117, a known conductivefilm can be used. For example, a single film of an element such astantalum (Ta), titanium (Ti), molybdenum (Mo), tungsten (W), chromium(Cr), or silicon (Si); a nitride film containing the aforementionedelement (typically, a tantalum nitride film, a tungsten nitride film, ora titanium nitride film); an alloy film in which the aforementionedelements are combined (typically, a Mo—W alloy or a Mo—Ta alloy); asilicide film containing the aforementioned element (typically, atungsten silicide film or a titanium silicide film); and the like can beused. Note that the aforementioned single film, nitride film, alloyfilm, silicide film, and the like can have a single-layer structure or astacked-layer structure.

An insulating film 110118 can have a single-layer structure or astacked-layer structure of an insulating film containing oxygen ornitrogen, such as silicon oxide (SiO_(x)), silicon nitride (SiN_(x)),silicon oxynitride (SiO_(x)N_(y)) (x>y), or silicon nitride oxide(SiN_(x)O_(y)) (x>y); or a film containing carbon, such as a DLC(diamond-like carbon), by a known method (such as a sputtering method ora plasma CVD method).

An insulating film 110119 can have a single-layer structure or astacked-layer structure of a siloxane resin; an insulating filmcontaining oxygen or nitrogen, such as silicon oxide (SiO_(x)), siliconnitride (SiN_(x)), silicon oxynitride (SiO_(x)N_(y)) (x>y), or siliconnitride oxide (SiN_(x)O_(y)) (x>y); a film containing carbon, such as aDLC (diamond-like carbon); or an organic material such as epoxy,polyimide, polyamide, polyvinyl phenol, benzocyclobutene, or acrylic.Note that a siloxane resin corresponds to a resin having Si—O—Si bonds.Siloxane includes a skeleton structure of a bond of silicon (Si) andoxygen (O). As a substituent, an organic group containing at leasthydrogen (such as an alkyl group or aromatic hydrocarbon) is used.Alternatively, a fluoro group, or a fluoro group and an organic groupcontaining at least hydrogen can be used as a substituent. Note that theinsulating film 110119 can be directly provided so as to cover the gateelectrode 110117 without provision of the insulating film 110118.

As a conductive film 110123, a single film of an element such as Al, Ni,C, W, Mo, Ti, Pt, Cu, Ta, Au, or Mn, a nitride film containing theaforementioned element, an alloy film in which the aforementionedelements are combined, a silicide film containing the aforementionedelement, or the like can be used. For example, as an alloy containing aplurality of the aforementioned elements, an Al alloy containing C andTi, an Al alloy containing Ni, an Al alloy containing C and Ni, an Alalloy containing C and Mn, or the like can be used. For example, whenthe conductive film has a stacked-layer structure, Al can be interposedbetween Mo, Ti, or the like; thus, resistance of Al to heat and chemicalreaction can be improved.

Next, with reference to the cross-sectional view of the plurality oftransistors each having a different structure shown in FIG. 28A,characteristics of each structure are described.

A transistor 110101 is a single drain transistor. Since the single draintransistor can be formed by a simple method, it is advantageous in lowmanufacturing cost and high yield. Here, the semiconductor layers 110113and 110115 have different concentrations of impurities. Thesemiconductor layer 110113 is used as a channel formation region. Thesemiconductor layers 110115 are used as a source region and a drainregion. By controlling the concentration of impurities in this manner,the resistivity of the semiconductor layer can be controlled. Moreover,an electrical connection state of the semiconductor layer and theconductive film 110123 can be closer to ohmic contact. Note that as amethod of separately forming the semiconductor layers each havingdifferent amount of impurities, a method can be used in which impuritiesare doped in a semiconductor layer using the gate electrode 110117 as amask.

A transistor 110102 is a transistor in which the gate electrode 110117is tapered at an angle of at least certain degrees. Since the transistorcan be formed by a simple method, it is advantageous in lowmanufacturing cost and high yield. Here, the semiconductor layers110113, 110114, and 110115 have different concentrations of impurities.The semiconductor layer 110113 is used as a channel region, thesemiconductor layers 110114 as lightly doped drain (LDD) regions, andthe semiconductor layers 110115 as a source region and a drain region.By controlling the amount of impurities in this manner, the resistivityof the semiconductor layer can be controlled. Moreover, an electricalconnection state of the semiconductor layer and the conductive film110123 can be closer to ohmic contact. Since the transistor includes theLDD regions, a high electric field is hardly applied inside thetransistor, so that deterioration of the element due to hot carriers canbe suppressed. Note that as a method of separately forming thesemiconductor layers having different amount of impurities, a method canbe used in which impurities are doped in a semiconductor layer using thegate electrode 110117 as a mask. In the transistor 110102, since thegate electrode 110117 is tapered at an angle of at least certaindegrees, gradient of the concentration of impurities doped in thesemiconductor layer through the gate electrode 110117 can be provided,and the LDD region can be easily formed.

A transistor 110103 is a transistor in which the gate electrode 110117is formed of at least two layers and a lower gate electrode is longerthan an upper gate electrode. In this specification, a shape of thelower and upper gate electrodes is called a hat shape. When the gateelectrode 110117 has a hat shape, an LDD region can be formed withoutaddition of a photomask. Note that a structure where the LDD regionoverlaps with the gate electrode 110117, like the transistor 110103, isparticularly called a GOLD (gate overlapped LDD) structure. As a methodof forming the gate electrode 110117 with a hat shape, the followingmethod may be used.

First, when the gate electrode 110117 is patterned, the lower and uppergate electrodes are etched by dry etching so that side surfaces thereofare inclined (tapered). Then, the inclination of the upper gateelectrode is processed to be almost perpendicular by anisotropicetching. Thus, the gate electrode a cross section of which is a hatshape is formed. After that, impurity elements are doped twice, so thatthe semiconductor layer 110113 used as the channel region, thesemiconductor layers 110114 used as the LDD regions, and thesemiconductor layers 110115 used as a source electrode and a drainelectrode are formed.

Note that part of the LDD region, which overlaps with the gate electrode110117, is referred to as an Lov region, and part of the LDD region,which does not overlap with the gate electrode 110117, is referred to asan Loff region. Here, the Loff region is highly effective in suppressingan off-current value, whereas it is not very effective in preventingdeterioration in an on-current value due to hot carriers by relieving anelectric field in the vicinity of the drain. On the other hand, the Lovregion is effective in preventing deterioration in the on-current valueby relieving the electric field in the vicinity of the drain, whereas itis not very effective in suppressing the off-current value. Thus, it ispreferable to form a transistor having a structure appropriate forcharacteristics of each of a variety of circuits. For example, when thesemiconductor device is used as a display device, a transistor having anLoff region is preferably used as a pixel transistor in order tosuppress the off-current value. On the other hand, as a transistor in aperipheral circuit, a transistor having an Lov region is preferably usedin order to prevent deterioration in the on-current value by relievingthe electric field in the vicinity of the drain.

A transistor 110104 is a transistor including a sidewall 110121 incontact with the side surface of the gate electrode 110117. When thetransistor includes the sidewall 110121, a region overlapping with thesidewall 110121 can be made to be an LDD region.

A transistor 110105 is a transistor in which an LDD (Loff) region isformed by performing doping of the semiconductor layer with the use of amask. Thus, the LDD region can surely be formed, and an off-currentvalue of the transistor can be reduced.

A transistor 110106 is a transistor in which an LDD (Lov) region isformed by performing doping of the semiconductor layer with the use of amask. Thus, the LDD region can surely be formed, and deterioration in anon-current value can be prevented by relieving the electric field in thevicinity of the drain of the transistor.

Next, an example of a method for manufacturing the transistor isdescribed with reference to FIGS. 28B to 28G.

Note that a structure and a manufacturing method of the transistor arenot limited to those in FIGS. 28A to 28G, and a variety of structuresand manufacturing methods can be used.

In this embodiment mode, a surface of the substrate 110111, a surface ofthe insulating film 110112, a surface of the semiconductor layer 110113,a surface of the semiconductor layer 110114, a surface of thesemiconductor layer 110115, a surface of the insulating film 110116, asurface of the insulating film 110118, or a surface of the insulatingfilm 110119 is oxidized or nitrided using plasma treatment, so that thesemiconductor layer or the insulating film can be oxidized or nitrided.By oxidizing or nitriding the semiconductor layer or the insulating filmby plasma treatment in such a manner, the surface of the semiconductorlayer or the insulating film is modified, and the insulating film can beformed to be denser than an insulating film formed by a CVD method or asputtering method. Thus, a defect such as a pinhole can be suppressed,and characteristics and the like of the semiconductor device can beimproved.

First, the surface of the substrate 110111 is washed using hydrofluoricacid (HF), alkaline, or pure water. The substrate 110111 can be a glasssubstrate using barium borosilicate glass, aluminoborosilicate glass, orthe like, a quartz substrate, a ceramic substrate, a metal substratecontaining stainless steel, or the like. Further, a substrate formed ofplastics typified by polyethylene terephthalate (PET), polyethylenenaphthalate (PEN), or polyethersulfone (PES), or a substrate formed of aflexible synthetic resin such as acrylic can also be used. Here, thecase where a glass substrate is used as the substrate 110111 is shown.

Here, an oxide film or a nitride film may be formed on the surface ofthe substrate 110111 by oxidizing or nitriding the surface of thesubstrate 110111 by plasma treatment (FIG. 28B). Hereinafter, aninsulating film such as an oxide film or a nitride film formed byperforming plasma treatment on the surface is also referred to as aplasma-treated insulating film. In FIG. 28B, an insulating film 110131is a plasma-treated insulating film. In general, when a semiconductorelement such as a thin film transistor is provided over a substrateformed of glass, plastic, or the like, an impurity element such asalkali metal (e.g., Na) or alkaline earth metal included in glass,plastic, or the like might be mixed into the semiconductor element sothat the semiconductor element is contaminated; thus, characteristics ofthe semiconductor element may be adversely affected in some cases.Nitridation of a surface of the substrate formed of glass, plastic, orthe like can prevent an impurity element such as alkali metal (e.g., Na)or alkaline earth metal included in the substrate from being mixed intothe semiconductor element.

When the surface is oxidized by plasma treatment, the plasma treatmentis performed in an oxygen atmosphere (e.g., in an atmosphere of oxygen(O₂) and a rare gas (containing at least one of He, Ne, Ar, Kr, and Xe),in an atmosphere of oxygen, hydrogen (H₂), and a rare gas, or in anatmosphere of dinitrogen monoxide and a rare gas). On the other hand,when the surface is nitrided by plasma treatment, the plasma treatmentis performed in a nitrogen atmosphere (e.g., in an atmosphere ofnitrogen (N₂) and a rare gas (containing at least one of He, Ne, Ar, Kr,and Xe), in an atmosphere of nitrogen, hydrogen, and a rare gas, or inan atmosphere of NH₃ and a rare gas). As a rare gas, Ar can be used, forexample. Alternatively, a gas in which Ar and Kr are mixed may be used.Accordingly, the plasma-treated insulating film contains a rare gas(containing at least one of He, Ne, Ar, Kr, and Xe) used for the plasmatreatment. For example, the plasma-treated insulating film contains Arwhen Ar is used.

It is preferable to perform plasma treatment in the atmospherecontaining the aforementioned gas, under conditions of an electrondensity in the range of 1×10¹¹ to 1×10¹³ cm⁻³ and a plasma electrontemperature in the range of 0.5 to 1.5 eV. Since the plasma electrondensity is high and the electron temperature in the vicinity of anobject to be treated is low, damage by plasma to the object to betreated can be prevented. Since the plasma electron density is as highas 1×10¹¹ cm⁻³ or more, an oxide film or a nitride film formed byoxidizing or nitriding the object to be treated by plasma treatment issuperior in its uniformity of thickness and the like as well as beingdense, as compared with a film formed by a CVD method, a sputteringmethod, or the like. Alternatively, since the plasma electrontemperature is as low as 1 eV or less, oxidation or nitridation can beperformed at a lower temperature as compared with that of a conventionalplasma treatment or thermal oxidation. For example, oxidation ornitridation can be performed sufficiently even when plasma treatment isperformed at a temperature lower than the strain point of a glasssubstrate by 100 degrees or more. Note that as frequency for generatingplasma, high frequency waves such as microwaves (2.45 GHz) can be used.Note that hereinafter, plasma treatment is performed using theaforementioned conditions unless otherwise specified.

Note that although FIG. 28B shows the case where the plasma-treatedinsulating film is formed by plasma treatment on the surface of thesubstrate 110111, this embodiment mode includes the case where aplasma-treated insulating film is not formed on the surface of thesubstrate 110111.

Note that although a plasma-treated insulating film formed by plasmatreatment on the surface of the object to be treated is not shown inFIGS. 28C to 28G; this embodiment mode includes the case where aplasma-treated insulating film formed by plasma treatment exists on thesurface of the substrate 110111, the insulating film 110112, thesemiconductor layer 110113, the semiconductor layer 110114, thesemiconductor layer 110115, the insulating film 110116, the insulatingfilm 110118, or the insulating film 110119.

Next, the insulating film 110112 is formed over the substrate 110111 bya known method (such as a sputtering method, an LPCVD method, or aplasma CVD method) (FIG. 28C). For the insulating film 110112, siliconoxide (SiO_(x)) or silicon oxynitride (SiO_(x)N_(y)) (x>y) can be used.

Here, a plasma-treated insulating film may be formed on the surface ofthe insulating film 110112 by oxidizing or nitriding the surface of theinsulating film 110112 by plasma treatment. By oxidizing the surface ofthe insulating film 110112, the surface of the insulating film 110112 ismodified, and a dense film with fewer defects such as a pinhole can beobtained. Further, by oxidizing the surface of the insulating film110112, the plasma-treated insulating film containing a little amount ofN atoms can be formed; thus, interface characteristics of theplasma-treated insulating film and a semiconductor layer are improvedwhen the semiconductor layer is provided over the plasma-treatedinsulating film. The plasma-treated insulating film contains a rare gas(containing at least one of He, Ne, Ar, Kr, and Xe) used for the plasmatreatment. Note that the plasma treatment can be performed in a similarmanner under the aforementioned conditions.

Next, the island-shaped semiconductor layers 110113 and 110114 areformed over the insulating film 110112 (FIG. 28D). The island-shapedsemiconductor layers 110113 and 110114 can be formed in such a mannerthat an amorphous semiconductor layer is formed over the insulating film110112 by using a material containing silicon (Si) as its main component(e.g., Si_(x)Ge_(1-x)) or the like by a known method (such as asputtering method, an LPCVD method, or a plasma CVD method), theamorphous semiconductor layer is crystallized, and the semiconductorlayer is selectively etched. Note that crystallization of the amorphoussemiconductor layer can be performed by a known crystallization methodsuch as a laser crystallization method, a thermal crystallization methodusing RTA or an annealing furnace, a thermal crystallization methodusing a metal element which promotes crystallization, or a method inwhich these methods are combined. Here, end portions of theisland-shaped semiconductor layers are provided with an angle of about90° (0=85 to 1000). Alternatively, the semiconductor layer 110114 to bea low concentration drain region may be formed by doping impurities withthe use of a mask.

Here, a plasma-treated insulating film may be formed on the surfaces ofthe semiconductor layers 110113 and 110114 by oxidizing or nitriding thesurfaces of the semiconductor layers 110113 and 110114 by plasmatreatment. For example, when Si is used for the semiconductor layers110113 and 110114, silicon oxide (SiO_(x)) or silicon nitride (SiN_(x))is formed as the plasma-treated insulating film. Alternatively, afterbeing oxidized by plasma treatment, the semiconductor layers 110113 and110114 may be nitrided by performing plasma treatment again. In thiscase, silicon oxide (Sio_(x)) is formed in contact with thesemiconductor layers 110113 and 110114, and silicon nitride oxide(SiN_(x)O_(y)) (x>y) is formed on the surface of the silicon oxide. Notethat when the semiconductor layer is oxidized by plasma treatment, theplasma treatment is performed in an oxygen atmosphere (e.g., in anatmosphere of oxygen (O₂) and a rare gas (containing at least one of He,Ne, Ar, Kr, and Xe), in an atmosphere of oxygen, hydrogen (H₂), and arare gas, or in an atmosphere of dinitrogen monoxide and a rare gas). Onthe other hand, when the semiconductor layer is nitrided by plasmatreatment, the plasma treatment is performed in a nitrogen atmosphere(e.g., in an atmosphere of nitrogen (N₂) and a rare gas (containing atleast one of He, Ne, Ar, Kr, and Xe), in an atmosphere of nitrogen,hydrogen, and a rare gas, or in an atmosphere of NH₃ and a rare gas). Asa rare gas, Ar can be used, for example. Alternatively, a gas in whichAr and Kr are mixed may be used. Accordingly, the plasma-treatedinsulating film contains a rare gas (containing at least one of He, Ne,Ar, Kr, and Xe) used for the plasma treatment. For example, theplasma-treated insulating film contains Ar when Ar is used.

Next, the insulating film 110116 is formed (FIG. 28E). The insulatingfilm 110116 can have a single-layer structure or a stacked-layerstructure of an insulating film containing oxygen or nitrogen, such assilicon oxide (SiO_(x)), silicon nitride (SiN_(x)), silicon oxynitride(SiO_(x)N_(y)) (x>y), or silicon nitride oxide (SiN_(x)O_(y)) (x>y), bya known method (such as a sputtering method, an LPCVD method, or aplasma CVD method). Note that when the plasma-treated insulating film isformed on the surfaces of the semiconductor layers 110113 and 110114 byperforming plasma treatment on the surfaces of the semiconductor layers110113 and 110114, the plasma-treated insulating film can be used as theinsulating film 110116.

Here, the surface of the insulating film 110116 may be oxidized ornitrided by plasma treatment, so that a plasma-treated insulating filmis formed on the surface of the insulating film 110116. Note that theplasma-treated insulating film contains a rare gas (containing at leastone of He, Ne, Ar, Kr, and Xe) used for the plasma treatment. The plasmatreatment can be performed in a similar manner under the aforementionedconditions.

Alternatively, after the insulating film 110116 is oxidized byperforming plasma treatment once in an oxygen atmosphere, the insulatingfilm 110116 may be nitrided by performing plasma treatment again in anitrogen atmosphere. By oxidizing or nitriding the surface of theinsulating film 110116 by plasma treatment in such a manner, the surfaceof the insulating film 110116 is modified, and a dense film can beformed. An insulating film obtained by plasma treatment is denser andhas fewer defects such as a pinhole, as compared with an insulating filmformed by a CVD method, a sputtering method, or the like. Thus,characteristics of a thin film transistor can be improved.

Next, the gate electrode 110117 is formed (FIG. 28F). The gate electrode110117 can be formed by a known method (such as a sputtering method, anLPCVD method, or a plasma CVD method).

In the transistor 110101, the semiconductor layers 110115 used as thesource region and the drain region can be formed by doping impuritiesafter the gate electrode 110117 is formed.

In the transistor 110102, the semiconductor layers 110114 used as theLDD regions and the semiconductor layers 110115 used as the sourceregion and the drain region can be formed by doping impurities after thegate electrode 110117 is formed.

In the transistor 110103, the semiconductor layers 110114 used as theLDD regions and the semiconductor layers 110115 used as the sourceregion and the drain region can be formed by doping impurities after thegate electrode 110117 is formed.

In the transistor 110104, the semiconductor layers 110114 used as theLDD regions and the semiconductor layers 110115 used as the sourceregion and the drain region can be formed by doping impurities after thesidewall 110121 is formed on the side surface of the gate electrode110117.

Note that silicon oxide (SiO_(x)) or silicon nitride (SiN_(x)) can beused for the sidewall 110121. As a method of forming the sidewall 110121on the side surface of the gate electrode 110117, a method can be used,for example, in which a silicon oxide (SiO_(x)) film or a siliconnitride (SiN_(x)) film is formed by a known method after the gateelectrode 110117 is formed, and then, the silicon oxide (SiO_(x)) filmor the silicon nitride (SiN_(x)) film is etched by anisotropic etching.Thus, the silicon oxide (SiO_(x)) film or the silicon nitride (SiN_(x))film remains only on the side surface of the gate electrode 110117, sothat the sidewall 110121 can be formed on the side surface of the gateelectrode 110117.

In the transistor 110105, the semiconductor layers 110114 used as theLDD (Loff) regions and the semiconductor layer 110115 used as the sourceregion and the drain region can be formed by doping impurities after amask 110122 is formed to cover the gate electrode 110117.

In the transistor 110106, the semiconductor layers 110114 used as theLDD (Lov) regions and the semiconductor layers 110115 used as the sourceregion and the drain region can be formed by doping impurities after thegate electrode 110117 is formed.

Next, the insulating film 110118 is formed (FIG. 28G). The insulatingfilm 110118 can have a single-layer structure or a stacked-layerstructure of an insulating film containing oxygen or nitrogen, such assilicon oxide (SiO_(x)), silicon nitride (SiN_(x)), silicon oxynitride(SiO_(x)N_(y)) (x>y), or silicon nitride oxide (SiN_(x)O_(y)) (x>y); ora film containing carbon, such as a DLC (diamond-like carbon), by aknown method (such as a sputtering method or a plasma CVD method).

Here, the surface of the insulating film 110118 may be oxidized ornitrided by plasma treatment, so that a plasma-treated insulating filmis formed on the surface of the insulating film 110118. Note that theplasma-treated insulating film contains a rare gas (containing at leastone of He, Ne, Ar, Kr, and Xe) used for the plasma treatment. The plasmatreatment can be performed in a similar manner under the aforementionedconditions.

Next, the insulating film 110119 is formed. The insulating film 110119can have a single-layer structure or a stacked-layer structure of anorganic material such as epoxy, polyimide, polyamide, polyvinyl phenol,benzocyclobutene, or acrylic; or a siloxane resin, in addition to aninsulating film containing oxygen or nitrogen, such as silicon oxide(SiO_(x)), silicon nitride (SiN_(x)), silicon oxynitride (SiO_(x)N_(y))(x>y), or silicon nitride oxide (SiN_(x)O_(y)) (x>y); or a filmcontaining carbon, such as a DLC (diamond-like carbon), by a knownmethod (such as a sputtering method or a plasma CVD method). Note that asiloxane resin corresponds to a resin having Si—O—Si bonds. Siloxaneincludes a skeleton structure of a bond of silicon (Si) and oxygen (O).As a substituent, an organic group containing at least hydrogen (such asan alkyl group or aromatic hydrocarbon) is used. Alternatively, a fluorogroup, or a fluoro group and an organic group containing at leasthydrogen can be used as a substituent. Note that the plasma-treatedinsulating film contains a rare gas (containing at least one of He, Ne,Ar, Kr, and Xe) used for the plasma treatment. For example, theplasma-treated insulating film contains Ar when Ar is used.

When an organic material such as polyimide, polyamide, polyvinyl phenol,benzocyclobutene, or acrylic, a siloxane resin, or the like is used forthe insulating film 110119, the surface of the insulating film 110119can be modified by oxidizing or nitriding the surface of the insulatingfilm by plasma treatment. Modification of the surface improves strengthof the insulating film 110119, and physical damage such as a crackgenerated when an opening is formed, for example, or film reduction inetching can be reduced. When the conductive film 110123 is formed overthe insulating film 110119, modification of the surface of theinsulating film 110119 improves adhesion to the conductive film. Forexample, when a siloxane resin is used for the insulating film 110119and nitrided by plasma treatment, a plasma-treated insulating filmcontaining nitrogen or a rare gas is formed by nitriding a surface ofthe siloxane resin, and physical strength is improved.

Next, a contact hole is formed in the insulating films 110119, 110118,and 110116 in order to form the conductive film 110123 which iselectrically connected to the semiconductor layer 110115. Note that thecontact hole may have a tapered shape. Thus, coverage with theconductive film 110123 can be improved.

FIG. 32 shows cross-sectional structures of a bottom-gate transistor anda capacitor.

A first insulating film (an insulating film 110502) is formed over anentire surface of a substrate 110501. The first insulating film canprevent impurities from the substrate from adversely affecting asemiconductor layer and changing properties of a transistor. That is,the first insulating film functions as a base film. Thus, a transistorwith high reliability can be formed. As the first insulating film, asingle layer or a stacked layer of a silicon oxide film, a siliconnitride film, a silicon oxynitride film (SiO_(x)N_(y)), or the like canbe used.

A first conductive layer (conductive layers 110503 and 110504) is formedover the first insulating film. The conductive layer 110503 includes aportion functioning as a gate electrode of a transistor 110520. Theconductive layer 110504 includes a portion functioning as a firstelectrode of a capacitor 110521. As the first conductive layer, anelement such as Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn,Fe, Ba, or Ge, or an alloy of these elements can be used. Alternatively,a stacked layer of these elements (including the alloy thereof) can beused.

A second insulating film (an insulating film 110514) is formed to coverat least the first conductive layer. The second insulating filmfunctions as a gate insulating film. As the second insulating film, asingle layer or a stacked layer of a silicon oxide film, a siliconnitride film, a silicon oxynitride film (SiO_(x)N_(y)), or the like canbe used.

Note that for a portion of the second insulating film, which is incontact with the semiconductor layer, a silicon oxide film is preferablyused. This is because the trap level at the interface between thesemiconductor layer and the second insulating film is lowered.

When the second insulating film is in contact with Mo, a silicon oxidefilm is preferably used for a portion of the second insulating film incontact with Mo. This is because the silicon oxide film does not oxidizeMo.

A semiconductor layer is formed in part of a portion over the secondinsulating film, which overlaps with the first conductive layer, by aphotolithography method, an inkjet method, a printing method, or thelike. Part of the semiconductor layer extends to a portion over thesecond insulating film, which does not overlap with the first conductivelayer. The semiconductor layer includes a channel formation region (achannel formation region 110510), an LDD region (LDD regions 110508 and110509), and an impurity region (impurity regions 110505, 110506, and110507). The channel formation region 110510 functions as a channelformation region of the transistor 110520. The LDD regions 110508 and110509 function as LDD regions of the transistor 110520. Note that theLDD regions 110508 and 110509 are not necessarily formed. The impurityregion 110505 includes a portion functioning as one of a sourceelectrode and a drain electrode of the transistor 110520. The impurityregion 110506 includes a portion functioning as the other of the sourceelectrode and the drain electrode of the transistor 110520. The impurityregion 110507 includes a portion functioning as a second electrode ofthe capacitor 110521.

A third insulating film (an insulating film 110511) is entirely formed.A contact hole is selectively formed in part of the third insulatingfilm. The insulating film 110511 functions as an interlayer film. As thethird insulating film, an inorganic material (e.g., silicon oxide,silicon nitride, or silicon oxynitride), an organic compound materialhaving a low dielectric constant (e.g., a photosensitive ornonphotosensitive organic resin material), or the like can be used.Alternatively, a material containing siloxane may be used. Note thatsiloxane is a material in which a skeleton structure is formed by a bondof silicon (Si) and oxygen (O). As a substitute, an organic groupcontaining at least hydrogen (such as an alkyl group or aromatichydrocarbon) is used. Alternatively, a fluoro group, or a fluoro groupand an organic group containing at least hydrogen may be used as asubstituent.

A second conductive layer (conductive layers 110512 and 110513) isformed over the third insulating film. The conductive layer 110512 isconnected to the other of the source electrode and the drain electrodeof the transistor 110520 through the contact hole formed in the thirdinsulating film. Thus, the conductive layer 110512 includes a portionfunctioning as the other of the source electrode and the drain electrodeof the transistor 110520. The conductive layer 110513 includes a portionfunctioning as the first electrode of the capacitor 110521. As thesecond conductive layer, an element such as Ti, Mo, Ta, Cr, W, Al, Nd,Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba, or Ge, or an alloy of these elementscan be used. Alternatively, a stacked layer of these elements (includingthe alloy thereof) can be used.

Note that in steps after forming the second conductive layer, variousinsulating films or various conductive films may be formed.

Next, structures of a transistor and a capacitor are described in thecase where an amorphous silicon (a-Si:H) film is used as a semiconductorlayer of the transistor.

FIG. 29 shows cross-sectional structures of a top-gate transistor and acapacitor.

A first insulating film (an insulating film 110202) is formed over anentire surface of a substrate 110201. The first insulating film canprevent impurities from the substrate from adversely affecting asemiconductor layer and changing properties of a transistor. That is,the first insulating film functions as a base film. Thus, a transistorwith high reliability can be formed. As the first insulating film, asingle layer or a stacked layer of a silicon oxide film, a siliconnitride film, a silicon oxynitride film (SiO_(x)N_(y)), or the like canbe used.

Note that the first insulating film is not necessarily formed. When thefirst insulating film is not formed, reduction in the number of stepsand reduction in manufacturing cost can be realized. Further, since thestructure can be simplified, the yield can be improved.

A first conductive layer (conductive layers 110203, 110204, and 110205)is formed over the first insulating film. The conductive layer 110203includes a portion functioning as one of a source electrode and a drainelectrode of a transistor 110220. The conductive layer 110204 includes aportion functioning as the other of the source electrode and the drainelectrode of the transistor 110220. The conductive layer 110205 includesa portion functioning as a first electrode of a capacitor 110221. As thefirst conductive layer, an element such as Ti, Mo, Ta, Cr, W, Al, Nd,Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba, or Ge, or an alloy of these elementscan be used. Alternatively, a stacked layer of these elements (includingthe alloy thereof) can be used.

A first semiconductor layer (semiconductor layers 110206 and 110207) isformed above the conductive layers 110203 and 110204. The semiconductorlayer 110206 includes a portion functioning as one of the sourceelectrode and the drain electrode. The semiconductor layer 110207includes a portion functioning as the other of the source electrode andthe drain electrode. As the first semiconductor layer, siliconcontaining phosphorus or the like can be used, for example.

A second semiconductor layer (a semiconductor layer 110208) is formedover the first insulating film and between the conductive layer 110203and the conductive layer 110204. Part of the semiconductor layer 110208extends over the conductive layers 110203 and 110204. The semiconductorlayer 110208 includes a portion functioning as a channel formationregion of the transistor 110220. As the second semiconductor layer, asemiconductor layer having no crystallinity such as an amorphous silicon(a-Si:H) layer, a semiconductor layer such as a microcrystallinesemiconductor (μ-Si:H) layer, or the like can be used.

A second insulating film (insulating films 110209 and 110210) is formedto cover at least the semiconductor layer 110208 and the conductivelayer 110205. The second insulating film functions as a gate insulatingfilm. As the second insulating film, a single layer or a stacked layerof a silicon oxide film, a silicon nitride film, a silicon oxynitridefilm (SiO_(x)N_(y)), or the like can be used.

Note that for a portion of the second insulating film, which is incontact with the second semiconductor layer, a silicon oxide film ispreferably used. This is because the trap level at the interface betweenthe second semiconductor layer and the second insulating film islowered.

When the second insulating film is in contact with Mo, a silicon oxidefilm is preferably used for a portion of the second insulating film incontact with Mo. This is because the silicon oxide film does not oxidizeMo.

A second conductive layer (conductive layers 110211 and 110212) isformed over the second insulating film. The conductive layer 110211includes a portion functioning as a gate electrode of the transistor110220. The conductive layer 110212 functions as a second electrode ofthe capacitor 110221 or a wiring. As the second conductive layer, anelement such as Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn,Fe, Ba, or Ge, or an alloy of these elements can be used. Alternatively,a stacked layer of these elements (including the alloy thereof) can beused.

Note that in steps after forming the second conductive layer, variousinsulating films or various conductive films may be formed.

FIG. 30 shows cross-sectional structures of an inversely staggered(bottom gate) transistor and a capacitor. In particular, the transistorshown in FIG. 30 has a channel-etched structure.

A first insulating film (an insulating film 110302) is formed over anentire surface of a substrate 110301. The first insulating film canprevent impurities from the substrate from adversely affecting asemiconductor layer and changing properties of a transistor. That is,the first insulating film functions as a base film. Thus, a transistorwith high reliability can be formed. As the first insulating film, asingle layer or a stacked layer of a silicon oxide film, a siliconnitride film, a silicon oxynitride film (SiO_(x)N_(y)), or the like canbe used.

Note that the first insulating film is not necessarily formed. When thefirst insulating film is not formed, reduction in the number of stepsand reduction in manufacturing cost can be realized. Further, since thestructure can be simplified, the yield can be improved.

A first conductive layer (conductive layers 110303 and 110304) is formedover the first insulating film. The conductive layer 110303 includes aportion functioning as a gate electrode of a transistor 110320. Theconductive layer 110304 includes a portion functioning as a firstelectrode of a capacitor 110321. As the first conductive layer, anelement such as Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn,Fe, Ba, or Ge, or an alloy of these elements can be used. Alternatively,a stacked layer of these elements (including the alloy thereof) can beused.

A second insulating film (an insulating film 110305) is formed to coverat least the first conductive layer. The second insulating filmfunctions as a gate insulating film. As the second insulating film, asingle layer or a stacked layer of a silicon oxide film, a siliconnitride film, a silicon oxynitride film (SiO_(x)N_(y)), or the like canbe used.

Note that for a portion of the second insulating film, which is incontact with the semiconductor layer, a silicon oxide film is preferablyused. This is because the trap level at the interface between thesemiconductor layer and the second insulating film is lowered.

When the second insulating film is in contact with Mo, a silicon oxidefilm is preferably used for a portion of the second insulating film incontact with Mo. This is because the silicon oxide film does not oxidizeMo.

A first semiconductor layer (a semiconductor layer 110306) is formed inpart of a portion over the second insulating film, which overlaps withthe first conductive layer, by a photolithography method, an inkjetmethod, a printing method, or the like. Part of the semiconductor layer110306 extends to a portion over the second insulating film, which doesnot overlap with the first conductive layer. The semiconductor layer110306 includes a portion functioning as a channel formation region ofthe transistor 110320. As the semiconductor layer 110306, asemiconductor layer having no crystallinity such as an amorphous silicon(a-Si:H) layer, a semiconductor layer such as a microcrystallinesemiconductor (μ-Si:H) layer, or the like can be used.

A second semiconductor layer (semiconductor layers 110307 and 110308) isformed over part of the first semiconductor layer. The semiconductorlayer 110307 includes a portion functioning as one of a source electrodeand a drain electrode. The semiconductor layer 110308 includes a portionfunctioning as the other of the source electrode and the drainelectrode. As the second semiconductor layer, silicon containingphosphorus or the like can be used, for example.

A second conductive layer (conductive layers 110309, 110310, and 110311)is formed over the second semiconductor layer and the second insulatingfilm. The conductive layer 110309 includes a portion functioning as oneof the source electrode and the drain electrode of the transistor110320. The conductive layer 110310 includes a portion functioning asthe other of the source electrode and the drain electrode of thetransistor 110320. The conductive layer 110311 includes a portionfunctioning as a second electrode of the capacitor 110321. As the secondconductive layer, an element such as Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag,Au, Pt, Nb, Si, Zn, Fe, Ba, or Ge, or an alloy of these elements can beused. Alternatively, a stacked layer of these elements (including thealloy thereof) can be used.

Note that in steps after forming the second conductive layer, variousinsulating films or various conductive films may be formed.

Here, an example of a step which is characteristic of the channel-etchedtype transistor is described. The first semiconductor layer and thesecond semiconductor layer can be formed using the same mask.Specifically, the first semiconductor layer and the second semiconductorlayer are continuously formed. Further, the first semiconductor layerand the second semiconductor layer are formed using the same mask.

Another example of a step which is characteristic of the channel-etchedtype transistor is described. The channel region of the transistor canbe formed without using an additional mask. Specifically, after thesecond conductive layer is formed, part of the second semiconductorlayer is removed using the second conductive layer as a mask.Alternatively, part of the second semiconductor layer is removed byusing the same mask as the second conductive layer. The firstsemiconductor layer below the removed second semiconductor layer servesas the channel formation region of the transistor.

FIG. 31 shows cross-sectional structures of an inversely staggered(bottom gate) transistor and a capacitor. In particular, the transistorshown in FIG. 31 has a channel protection (channel stop) structure.

A first insulating film (an insulating film 110402) is formed over anentire surface of a substrate 110401. The first insulating film canprevent impurities from the substrate from adversely affecting asemiconductor layer and changing properties of a transistor. That is,the first insulating film functions as a base film. Thus, a transistorwith high reliability can be formed. As the first insulating film, asingle layer or a stacked layer of a silicon oxide film, a siliconnitride film, a silicon oxynitride film (SiO_(x)N_(y)), or the like canbe used.

Note that the first insulating film is not necessarily formed. When thefirst insulating film is not formed, reduction in the number of stepsand reduction in manufacturing cost can be realized. Further, since thestructure can be simplified, the yield can be improved.

A first conductive layer (conductive layers 110403 and 110404) is formedover the first insulating film. The conductive layer 110403 includes aportion functioning as a gate electrode of a transistor 110420. Theconductive layer 110404 includes a portion functioning as a firstelectrode of a capacitor 110421. As the first conductive layer, anelement such as Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn,Fe, Ba, or Ge, or an alloy of these elements can be used. Alternately, astacked layer of these elements (including the alloy thereof) can beused.

A second insulating film (an insulating film 110405) is formed to coverat least the first conductive layer. The second insulating filmfunctions as a gate insulating film. As the second insulating film, asingle layer or a stacked layer of a silicon oxide film, a siliconnitride film, a silicon oxynitride film (SiO_(x)N_(y)), or the like canbe used.

Note that for a portion of the second insulating film, which is incontact with the semiconductor layer, a silicon oxide film is preferablyused. This is because the trap level at the interface between thesemiconductor layer and the second insulating film is lowered.

When the second insulating film is in contact with Mo, a silicon oxidefilm is preferably used for a portion of the second insulating film incontact with Mo. This is because the silicon oxide film does not oxidizeMo.

A first semiconductor layer (a semiconductor layer 110406) is formed inpart of a portion over the second insulating film, which overlaps withthe first conductive layer, by a photolithography method, an inkjetmethod, a printing method, or the like. Part of the semiconductor layer110406 extends to a portion over the second insulating film, which doesnot overlap with the first conductive layer. The semiconductor layer110406 includes a portion functioning as a channel formation region ofthe transistor 110420. As the semiconductor layer 110406, asemiconductor layer having no crystallinity such as an amorphous silicon(a-Si:H) layer, a semiconductor layer such as a microcrystallinesemiconductor (μ-Si:H) layer, or the like can be used.

A third insulating film (an insulating film 110412) is formed over partof the first semiconductor layer. The insulating film 110412 preventsthe channel region of the transistor 110420 from being removed byetching. That is, the insulating film 110412 functions as a channelprotection film (a channel stop film). As the third insulating film, asingle layer or a stacked layer of a silicon oxide film, a siliconnitride film, a silicon oxynitride film (SiO_(x)N_(y)), or the like canbe used.

A second semiconductor layer (semiconductor layers 110407 and 110408) isformed over part of the first semiconductor layer and part of the thirdinsulating film. The semiconductor layer 110407 includes a portionfunctioning as one of a source electrode and a drain electrode. Thesemiconductor layer 110408 includes a portion functioning as the otherof the source electrode and the drain electrode. As the secondsemiconductor layer, silicon containing phosphorus or the like can beused, for example.

A second conductive layer (conductive layers 110409, 110410, and 110411)is formed over the second semiconductor layer. The conductive layer110409 includes a portion functioning as one of the source electrode andthe drain electrode of the transistor 110420. The conductive layer110410 includes a portion functioning as the other of the sourceelectrode and the drain electrode of the transistor 110420. Theconductive layer 110411 includes a portion functioning as a secondelectrode of the capacitor 110421. As the second conductive layer, anelement such as Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn,Fe, Ba, or Ge, or an alloy of these elements can be used. Alternately, astacked layer of these elements (including the alloy thereof) can beused.

Note that in steps after forming the second conductive layer, variousinsulating films or various conductive films may be formed.

Here, an example of a step which is characteristic of the channelprotection type transistor is described. The first semiconductor layer,the second semiconductor layer, and the second conductive layer can beformed using the same mask. At the same time, the channel formationregion can be formed. Specifically, the first semiconductor layer isformed, and then, the third insulating film (i.e., the channelprotection film or the channel stop film) is patterned using a mask.Next, the second semiconductor layer and the second conductive layer arecontinuously formed. Then, after the second conductive layer is formed,the first semiconductor layer, the second semiconductor layer, and thesecond conductive film are patterned using the same mask. Note that partof the first semiconductor layer below the third insulating film isprotected by the third insulating film, and thus is not removed byetching. This part (a part of the first semiconductor layer over whichthe third insulating film is formed) serves as the channel region.

Next, an example where a semiconductor substrate is used as a substratefor a transistor is described. Since a transistor formed using asemiconductor substrate has high mobility, the size of the transistorcan be decreased. Accordingly, the number of transistors per unit areacan be increased (the degree of integration can be improved), and thesize of the substrate can be decreased as the degree of integration isincreased in the case of the same circuit structure. Thus, manufacturingcost can be reduced. Further, since the circuit scale can be increasedas the degree of integration is increased in the case of the samesubstrate size, more advanced functions can be provided without increasein manufacturing cost. Moreover, reduction in variations incharacteristics can improve manufacturing yield. Reduction in operatingvoltage can reduce power consumption. High mobility can realizehigh-speed operation.

When a circuit which is formed by integrating transistors formed using asemiconductor substrate is mounted on a device in the form of an IC chipor the like, the device can be provided with a variety of functions. Forexample, when a peripheral driver circuit (e.g., a data driver (a sourcedriver), a scan driver (a gate driver), a timing controller, an imageprocessing circuit, an interface circuit, a power supply circuit, or anoscillation circuit) of a display device is formed by integratingtransistors formed using a semiconductor substrate, a small peripheralcircuit which can be operated with low power consumption and at highspeed can be formed at low cost in high yield. Note that a circuit whichis formed by integrating transistors formed using a semiconductorsubstrate may include a unipolar transistor. Thus, a manufacturingprocess can be simplified, so that manufacturing cost can be reduced.

A circuit which is formed by integrating transistors formed using asemiconductor substrate may also be used for a display panel, forexample. More specifically, the circuit can be used for a reflectiveliquid crystal panel such as a liquid crystal on silicon (LCOS) device,a digital micromirror device (DMD) in which micromirrors are integrated,an EL panel, and the like. When such a display panel is formed using asemiconductor substrate, a small display panel which can be operatedwith low power consumption and at high speed can be formed at low costin high yield. Note that the display panel may be formed over an elementhaving a function other than a function of driving the display panel,such as a large-scale integration (LSI).

Hereinafter, a method of forming a transistor using a semiconductorsubstrate is described.

First, element isolation regions 110604 and 110606 (hereinafter,referred to as regions 110604 and 110606) are formed on a semiconductorsubstrate 110600 (see FIG. 33A). The regions 110604 and 110606 providedin the semiconductor substrate 110600 are isolated from each other by aninsulating film 110602. The example shown here is the case where asingle crystal Si substrate having n-type conductivity is used as thesemiconductor substrate 110600, and a p-well 110607 is provided in theregion 110606 of the semiconductor substrate 110600.

Any substrate can be used as the substrate 110600 as long as it is asemiconductor substrate. For example, a single crystal Si substratehaving n-type or p-type conductivity, a compound semiconductor substrate(e.g., a GaAs substrate, an InP substrate, a GaN substrate, a SiCsubstrate, a sapphire substrate, or a ZnSe substrate), an SOI (siliconon insulator) substrate formed by a bonding method or a SIMOX(separation by implanted oxygen) method, or the like can be used.

The regions 110604 and 110606 can be formed by a LOCOS (local oxidationof silicon) method, a trench isolation method, or the like asappropriate.

The p-well formed in the region 110606 of the semiconductor substrate110600 can be formed by selective doping of the semiconductor substrate110600 with a p-type impurity element. As the p-type impurity element,boron (B), aluminum (Al), gallium (Ga), or the like can be used.

Note that in this embodiment mode, although the region 110604 is notdoped with an impurity element because a semiconductor substrate havingn-type conductivity is used as the semiconductor substrate 110600, ann-well may be formed in the region 110604 by introduction of an n-typeimpurity element. As the n-type impurity element, phosphorus (P),arsenic (As), or the like can be used. In contrast, when a semiconductorsubstrate having p-type conductivity is used, the region 110604 may bedoped with an n-type impurity element to form an n-well, whereas theregion 110606 may be doped with no impurity element.

Next, insulating films 110632 and 110634 are formed so as to cover theregions 110604 and 110606, respectively (see FIG. 33B).

For example, surfaces of the regions 110604 and 110606 provided in thesemiconductor substrate 110600 are oxidized by heat treatment, so thatthe insulating films 110632 and 110634 can be formed of silicon oxidefilms. Alternatively, the insulating films 110632 and 110634 may beformed to have a stacked-layer structure of a silicon oxide film and afilm containing oxygen and nitrogen (a silicon oxynitride film) byforming a silicon oxide film by a thermal oxidation method and thennitriding the surface of the silicon oxide film by nitridationtreatment.

Further alternatively, the insulating films 110632 and 110634 may beformed by plasma treatment as described above. For example, theinsulating films 110632 and 110634 can be formed using a silicon oxide(SiO_(x)) film or a silicon nitride (SiN_(x)) film obtained byapplication of high-density plasma oxidation treatment or high-densityplasma nitridation treatment to the surfaces of the regions 110604 and110606 provided in the semiconductor substrate 110600. As anotherexample, after application of high-density plasma oxidation treatment tothe surfaces of the regions 110604 and 110606, high-density plasmanitridation treatment may be performed. In that case, silicon oxidefilms are formed on the surfaces of the regions 110604 and 110606, andthen silicon oxynitride films are formed on the silicon oxide films.Thus, each of the insulating films 110632 and 110634 is formed to have astacked-layer structure of the silicon oxide film and the siliconoxynitride film. As another example, after silicon oxide films areformed on the surfaces of the regions 110604 and 110606 by a thermaloxidation method, high-density plasma oxidation treatment orhigh-density nitridation treatment may be applied to the silicon oxidefilms.

The insulating films 110632 and 110634 formed over the regions 110604and 110606 of the semiconductor substrate 110600 function as the gateinsulating films of transistors which are completed later.

Next, a conductive film is formed so as to cover the insulating films110632 and 110634 which are formed over the regions 110604 and 110606,respectively (see FIG. 33C). Here, an example is shown in which theconductive film is formed by sequentially stacking conductive films110636 and 110638. It is needless to say that the conductive film may beformed using a single-layer structure or a stacked-layer structure ofthree or more layers.

As a material of the conductive films 110636 and 110638, an elementselected from tantalum (Ta), tungsten (W), titanium (Ti), molybdenum(Mo), aluminum (Al), copper (Cu), chromium (Cr), niobium (Nb), and thelike, or an alloy material or a compound material containing such anelement as its main component can be used. Alternatively, a metalnitride film obtained by nitridation of the above element can be used.Further alternatively, a semiconductor material typified bypolycrystalline silicon doped with an impurity element such asphosphorus or silicide in which a metal material is introduced can beused.

In this case, a stacked-layer structure is employed in which tantalumnitride is used for the conductive film 110636 and tungsten is used forthe conductive film 110638. Alternatively, it is also possible to formthe conductive film 110636 using a single-layer film or a stacked-layerfilm of tungsten nitride, molybdenum nitride, and/or titanium nitride.For the conductive film 110638, it is possible to use a single-layerfilm or a stacked-layer film of tantalum, molybdenum, and/or titanium.

Next, the stacked conductive films 110636 and 110638 are selectivelyremoved by etching, so that the conductive films 110636 and 110638remain above part of the regions 110604 and 110606, respectively. Thus,gate electrodes 110640 and 110642 are formed (see FIG. 34A).

Next, a resist mask 110648 is selectively formed so as to cover theregion 110604, and the region 110606 is doped with an impurity element,using the resist mask 110648 and the gate electrode 110642 as masks;thus, impurity regions 110652 are formed (see FIG. 34B). As an impurityelement, an n-type impurity element or a p-type impurity element isused. As the n-type impurity element, phosphorus (P), arsenic (As), orthe like can be used. As the p-type impurity element, boron (B),aluminum (Al), gallium (Ga), or the like can be used. Here, phosphorus(P) is used as the impurity element. Note that after the impurityelement is introduced, heat treatment may be performed in order todisperse the impurity element and to recover the crystalline structure.

In FIG. 34B, by introduction of an impurity element, impurity regions110652 which form source and drain regions and a channel formationregion 110650 are formed in the region 110606.

Next, a resist mask 110666 is selectively formed so as to cover theregion 110606, and the region 110604 is doped with an impurity element,using the resist mask 110666 and the gate electrode 110640 as masks;thus, impurity regions 110670 are formed (see FIG. 34C). As the impurityelement, an n-type impurity element or a p-type impurity element isused. As the n-type impurity element, phosphorus (P), arsenic (As), orthe like can be used. As the p-type impurity element, boron (B),aluminum (Al), gallium (Ga), or the like can be used. At this time, animpurity element (e.g., boron (B)) of a conductivity type different fromthat of the impurity element introduced into the region 110606 in FIG.34B is used. As a result, the impurity regions 110670 which form sourceand drain regions and a channel formation region 110668 are formed inthe region 110604. Note that after the impurity element is introduced,heat treatment may be performed in order to disperse the impurityelement and to recover the crystalline structure.

Next, a second insulating film 110672 is formed so as to cover theinsulating films 110632 and 110634 and the gate electrodes 110640 and110642. Further, wirings 110674 which are electrically connected to theimpurity regions 110652 and 110670 formed in the regions 110606 and110604 respectively are formed (see FIG. 34D).

The second insulating film 110672 can be formed to have a single-layerstructure or a stacked-layer structure of an insulating film containingoxygen and/or nitrogen such as silicon oxide (SiO_(x)), silicon nitride(SiN_(x)), silicon oxynitride (SiO_(x)N_(y)) (x>y), or silicon nitrideoxide (SiN_(x)O_(y)) (x>y); a film containing carbon such as DLC(diamond-like carbon); an organic material such as epoxy, polyimide,polyamide, polyvinyl phenol, benzocyclobutene, or acrylic; or a siloxanematerial such as a siloxane resin by a CVD method, a sputtering method,or the like. A siloxane material corresponds to a material having a bondof Si—O—Si. Siloxane has a skeleton structure with the bond of silicon(Si) and oxygen (O). As a substituent of siloxane, an organic groupcontaining at least hydrogen (e.g., an alkyl group or aromatichydrocarbon) is used. Alternatively, a fluoro group, or both a fluorogroup and an organic group containing at least hydrogen may be used asthe substituent.

The wirings 110674 are formed with a single layer or a stacked layer ofan element selected from aluminum (Al), tungsten (W), titanium (Ti),tantalum (Ta), molybdenum (Mo), nickel (Ni), platinum (Pt), copper (Cu),gold (Au), silver (Ag), manganese (Mn), neodymium (Nd), carbon (C), andsilicon (Si), or an alloy material or a compound material containingsuch an element as its main component by a CVD method, a sputteringmethod, or the like. An alloy material containing aluminum as its maincomponent corresponds to, for example, a material which containsaluminum as its main component and also contains nickel, or a materialwhich contains aluminum as its main component and also contains nickeland one or both of carbon and silicon. The wirings 110674 are preferablyformed to have a stacked-layer structure of a barrier film, analuminum-silicon (Al—Si) film, and a barrier film or a stacked-layerstructure of a barrier film, an aluminum-silicon (Al—Si) film, atitanium nitride film, and a barrier film. Note that the barrier filmcorresponds to a thin film formed of titanium, titanium nitride,molybdenum, or molybdenum nitride. Aluminum and aluminum silicon aresuitable materials for forming the wirings 110674 because they have highresistance values and are inexpensive. For example, when barrier layersare provided as the top layer and the bottom layer, generation ofhillocks of aluminum or aluminum silicon can be prevented. For example,when a barrier film is formed of titanium which is an element having ahigh reducing property, even if a thin natural oxide film is formed on acrystalline semiconductor film, the natural oxide film can be reduced.As a result, the wirings 110674 can be connected to the crystallinesemiconductor in electrically and physically favorable condition.

Note that the structure of a transistor is not limited to that shown inthe drawing. For example, a transistor with an inversely staggeredstructure, a FinFET structure, or the like can be used. A FinFETstructure is preferable because it can suppress a short channel effectwhich occurs along with reduction in transistor size.

Next, another example in which a semiconductor substrate is used as asubstrate for forming a transistor is described.

First, an insulating film is formed on a substrate 110800. Here, asingle crystal Si having n-type conductivity is used for the substrate110800, and insulating films 110802 and 110804 are formed on thesubstrate 110800 (see FIG. 35A). For example, silicon oxide (SiO_(x)) isformed for the insulating film 110802 by performing heat treatment onthe substrate 110800. Moreover, silicon nitride (SiN_(x)) is formed by aCVD method or the like.

Any substrate can be used as the substrate 110800 as long as it is asemiconductor substrate. For example, a single crystal Si substratehaving n-type or p-type conductivity, a compound semiconductor substrate(e.g., a GaAs substrate, an InP substrate, a GaN substrate, a SiCsubstrate, a sapphire substrate, or a ZnSe substrate), an SOI (siliconon insulator) substrate formed by a bonding method or a SIMOX(separation by implanted oxygen) method, or the like can be used.

The insulating film 110804 may be provided by forming the insulatingfilm 110802 and then nitriding the insulating film 110802 byhigh-density plasma treatment. Note that the insulating film may have asingle-layer structure or a stacked-layer structure of three or morelayers.

Next, a pattern of a resist mask 110806 is selectively formed. Then,etching is selectively performed using the resist mask 110806 as a mask,whereby depressed portions 110808 are selectively formed in thesubstrate 110800 (see FIG. 35B). The substrate 110800 and the insulatingfilms 110802 and 110804 can be etched by dry etching using plasma.

Next, after the pattern of the resist mask 110806 is removed, aninsulating film 110810 is formed so as to fill the depressed portions110808 formed in the substrate 110800 (see FIG. 35C).

The insulating film 110810 is formed using an insulating material suchas silicon oxide, silicon nitride, silicon oxynitride (SiO_(x)N_(y))(x>y>0), or silicon nitride oxide (SiN_(x)O_(y)) (x>y>0) by a CVDmethod, a sputtering method, or the like. Here, as the insulating film110810, a silicon oxide film is formed using a tetraethyl orthosilicate(TEOS) gas by an atmospheric pressure CVD method or a low pressure CVDmethod.

Next, a surface of the substrate 110800 is exposed by performinggrinding treatment, polishing treatment, or chemical mechanicalpolishing (CMP) treatment. Then, the surface of the substrate 110800 isseparated by insulating films 110810 formed in the depressed portions110808 of the substrate 110800. Here, the separated regions are referredto as regions 110812 and 110813 (see FIG. 36A). Note that the insulatingfilms 110810 are obtained by partial removal of the insulating films110810 by grinding treatment, polishing treatment, or CMP treatment.

Subsequently, the p-well can be formed in the region 110813 of thesemiconductor substrate 110800 by selective introduction of an impurityelement having p-type conductivity. As the p-type impurity element,boron (B), aluminum (Al), gallium (Ga), or the like can be used. Here,as the impurity element, boron (B) is introduced into the region 110813.Note that after the impurity element is introduced, heat treatment maybe performed in order to disperse the impurity element and to recoverthe crystalline structure.

Although an impurity element is not necessarily introduced into theregion 110812 when a semiconductor substrate having n-type conductivityis used as the semiconductor substrate 110800, an n-well may be formedin the region 110812 by introduction of an n-type impurity element. Asthe n-type impurity element, phosphorus (P), arsenic (As), or the likecan be used.

In contrast, when a semiconductor substrate having p-type conductivityis used, the region 110812 may be doped with an n-type impurity elementto form an n-well, whereas the region 110813 may be doped with noimpurity element.

Next, insulating films 110832 and 110834 are formed, respectively, onthe surfaces of the regions 110812 and 110813 of the substrate 110800(see FIG. 36B).

For example, the surfaces of the regions 110812 and 110813 provided inthe semiconductor substrate 110800 are oxidized by heat treatment, sothat the insulating films 110832 and 110834 can be formed of siliconoxide films. Alternatively, the insulating films 110832 and 110834 maybe formed to have a stacked-layer structure of a silicon oxide film anda film containing oxygen and nitrogen (a silicon oxynitride film) by theforming a silicon oxide film by a thermal oxidation method and thennitriding the surface of the silicon oxide film by nitridationtreatment.

Further alternatively, the insulating films 110832 and 110834 may beformed by plasma treatment as described above. For example, theinsulating films 110832 and 110834 can be formed using a silicon oxide(SiO_(x)) film or a silicon nitride (SiN_(x)) film obtained byapplication of high-density plasma oxidation treatment or high-densityplasma nitridation treatment to the surfaces of the regions 110812 and110813 provided in the substrate 110800. As another example, afterapplication of high-density plasma oxidation treatment to the surfacesof the regions 110812 and 110813, high-density plasma nitridationtreatment may be performed. In that case, silicon oxide films are formedon the surfaces of the regions 110812 and 110813, and then siliconoxynitride films are formed on the silicon oxide films. Thus, each ofthe insulating films 110832 and 110834 is formed to have a stacked-layerstructure of the silicon oxide film and the silicon oxynitride film. Asanother example, after silicon oxide films are formed on the surfaces ofthe regions 110812 and 110813 by a thermal oxidation method,high-density plasma oxidation treatment or high-density nitridationtreatment may be applied to the silicon oxide films.

The insulating films 110832 and 110834 formed over the regions 110812and 110813 of the semiconductor substrate 110800 function as the gateinsulating films of transistors which are completed later.

Next, a conductive film is formed so as to cover the insulating films110832 and 110834 which are formed over the regions 110812 and 110813,respectively, provided in the substrate 110800 (see FIG. 36C). Here, anexample is shown in which the conductive film is formed by sequentiallystacking conductive films 110836 and 110838. It is needless to say thatthe conductive film may be formed using a single-layer structure or astacked-layer structure of three or more layers.

As a material of the conductive films 110836 and 110838, an elementselected from tantalum (Ta), tungsten (W), titanium (Ti), molybdenum(Mo), aluminum (Al), copper (Cu), chromium (Cr), niobium (Nb), and thelike, or an alloy material or a compound material containing such anelement as its main component can be used. Alternatively, a metalnitride film obtained by nitridation of the above element can be used.Further alternatively, a semiconductor material typified bypolycrystalline silicon doped with an impurity element such asphosphorus or silicide in which a metal material is introduced can beused.

In this case, a stacked-layer structure is employed in which tantalumnitride is used for the conductive film 110836 and tungsten is used forthe conductive film 110838. Alternatively, it is also possible to formthe conductive film 110836 using a single-layer film or a stacked-layerfilm of tantalum nitride, tungsten nitride, molybdenum nitride, and/ortitanium nitride. For the conductive film 110838, it is possible to usea single-layer film or a stacked-layer film of tungsten, tantalum,molybdenum, and/or titanium.

Next, the stacked conductive films 110836 and 110838 are selectivelyremoved by etching, so that the conductive films 110836 and 110838remain above part of the regions 110812 and 110813 of the substrate110800, respectively. Thus, conductive films 110840 and 110842functioning as gate electrodes are formed (see FIG. 36D). Here, thesurface of the substrate 110800 is made to be exposed in the regionwhich does not overlap with the conductive films 110840 and 110842.

Specifically, in the region 110812 of the substrate 110800, a portion ofthe insulating film 110832 which does not overlap with the conductivefilm 110840 is selectively removed, and an end portion of the conductivefilm 110840 and an end portion of the insulating film 110832 are made toroughly match. Further, in the region 110813 of the substrate 110800,part of the insulating film 110834 which does not overlap with theconductive film 110842 is selectively removed, and an end portion of theconductive film 110842 and an end portion of the insulating film 110834are made to roughly match.

In this case, insulating films and the like of the portions which do notoverlap with the conductive films 110840 and 110842 may be removed atthe same time as formation of the conductive films 110840 and 110842.Alternatively, the insulating films and the like of the portions whichdo not overlap may be removed using the resist mask, which is left afterthe conductive films 110840 and 110842 are formed, or the conductivefilms 110840 and 110842 as masks.

Next, an impurity element is selectively introduced into the regions110812 and 110813 of the substrate 110800 (see FIG. 37A). Here, ann-type impurity element is selectively introduced into the region 110813at a low concentration, using the conductive film 110842 as a mask. Onthe other hand, a p-type impurity element is selectively introduced intothe region 110812 at a low concentration, using the conductive film110840 as a mask. As the n-type impurity element, phosphorus (P),arsenic (As), or the like can be used. As the p-type impurity element,boron (B), aluminum (Al), gallium (Ga), or the like can be used. Notethat after the impurity element is introduced, heat treatment may beperformed in order to disperse the impurity element and to recover thecrystalline structure.

Next, sidewalls 110854 which are in contact with side surfaces of theconductive films 110840 and 110842 are formed. Specifically, thesidewalls are formed to have a single-layer structure or a stacked-layerstructure of a film containing an inorganic material such as silicon,oxide of silicon, or nitride of silicon, or a film containing an organicmaterial such as an organic resin by a plasma CVD method, a sputteringmethod, or the like. Then, the insulating films are selectively etchedby anisotropic etching mainly in a perpendicular direction, so that thesidewalls are formed in contact with the side surfaces of the conductivefilms 110840 and 110842. Note that the sidewalls 110854 are used asmasks for doping in forming LDD (lightly doped drain) regions. Here, thesidewalls 110854 are formed to be also in contact with side surfaces ofthe insulating films or floating gate electrodes formed under theconductive films 110840 and 110842.

Subsequently, an impurity element is introduced into the regions 110812and 110813 of the substrate 110800, using the sidewalls 110854 and theconductive films 110840 and 110842 as masks; thus, impurity regionsfunctioning as source and drain regions are formed (see FIG. 37B). Here,an n-type impurity element is introduced into the region 110813 of thesubstrate 110800 at a high concentration, using the sidewalls 110854 andthe conductive film 110842 as masks, and a p-type impurity element isintroduced into the region 110812 at a high concentration, using thesidewalls 110854 and the conductive film 110840 as masks.

As a result, in the region 110812 of the substrate 110800, an impurityregion 110858 forming a source or drain region, a low-concentrationimpurity region 110860 forming an LDD region, and a channel formationregion 110856 are formed. Moreover, in the region 110813 of thesubstrate 110800, an impurity region 110864 forming a source or drainregion, a low-concentration impurity region 110866 forming an LDDregion, and a channel formation region 110862 are formed.

Note that although the example in which the LDD regions are formed usingthe sidewalls is described, the invention is not limited thereto. TheLDD regions may be formed using a mask or the like without the use ofthe sidewalls, or is not necessarily formed. When the LDD regions arenot formed, a manufacturing process can be simplified, so thatmanufacturing cost can be reduced.

Note that in this embodiment mode, impurity elements are introduced in astate where the surface of the substrate 110800 is exposed in the regionwhich does not overlap with the conductive films 110840 and 110842.Accordingly, the channel formation regions 110856 and 110862 formed inthe regions 110812 and 110813 respectively of the substrate 110800 canbe formed in a self-aligned manner with the conductive films 110840 and110842, respectively.

Next, a second insulating film 110877 is formed so as to cover theinsulating films, conductive films, and the like provided over theregions 110812 and 110813 of the substrate 110800, and openings 110878are formed in the insulating film 110877 (see FIG. 37C).

The second insulating film 110877 can be formed to have a single-layerstructure or a stacked-layer structure of an insulating film containingoxygen and/or nitrogen such as silicon oxide (SiO_(x)), silicon nitride(SiN_(x)), silicon oxynitride (SiO_(x)N_(y)) (x>y), or silicon nitrideoxide (SiN_(x)O_(y)) (x>y); a film containing carbon such asdiamond-like carbon (DLC); an organic material such as epoxy, polyimide,polyamide, polyvinyl phenol, benzocyclobutene, or acrylic; or a siloxanematerial such as a siloxane resin by a CVD method, a sputtering method,or the like. A siloxane material corresponds to a material having a bondof Si—O—Si. Siloxane has a skeleton structure with the bond of silicon(Si) and oxygen (O). As a substituent of siloxane, an organic groupcontaining at least hydrogen (for example, an alkyl group or aromatichydrocarbon) is used. Alternatively, a fluoro group, or both a fluorogroup and an organic group containing at least hydrogen may be used asthe substituent.

Next, a conductive film 110880 is formed in each of the openings 110878by a CVD method, and conductive films 110882 a to 110882 d areselectively formed over the insulating film 110877 so as to beelectrically connected to the conductive films 110880 (see FIG. 37D).

The conductive films 110880 and 110882 a to 110882 d are formed to havea single-layer structure or a stacked-layer structure of an elementselected from aluminum (Al), tungsten (W), titanium (Ti), tantalum (Ta),molybdenum (Mo), nickel (Ni), platinum (Pt), copper (Cu), gold (Au),silver (Ag), manganese (Mn), neodymium (Nd), carbon (C), and silicon(Si), or an alloy material or a compound material containing such anelement as its main component by a CVD method, a sputtering method, orthe like. An alloy material containing aluminum as its main componentcorresponds to, for example, a material which contains aluminum as itsmain component and also contains nickel, or a material which containsaluminum as its main component and also contains nickel and one or bothof carbon and silicon. The conductive films 110880 and 110882 a to110882 d are preferably formed to have a stacked-layer structure of abarrier film, an aluminum-silicon (Al—Si) film, and a barrier film or astacked structure of a barrier film, an aluminum-silicon (Al—Si) film, atitanium nitride film, and a barrier film. Note that the barrier filmcorresponds to a thin film formed of titanium, titanium nitride,molybdenum, or molybdenum nitride. Aluminum and aluminum silicon aresuitable materials for forming the conductive film 110880 because theyhave high resistance values and are inexpensive. For example, whenbarrier layers are provided as the top layer and the bottom layer,generation of hillocks of aluminum or aluminum silicon can be prevented.For example, when a barrier film is formed of titanium which is anelement having a high reducing property, even if a thin natural oxidefilm is formed on the crystalline semiconductor film, the natural oxidefilm can be reduced, and a favorable contact between the conductive filmand the crystalline semiconductor film can be obtained. Here, theconductive films 110880 can be formed by selective growth of tungsten(W) by a CVD method.

By the steps described above, a p-channel transistor formed in theregion 110812 of the substrate 110800 and an n-channel transistor formedin the region 110813 of the substrate 110800 can be obtained.

Note that the structure of a transistor is not limited to that shown inthe drawing. For example, a transistor with an inversely staggeredstructure, a FinFET structure, or the like can be used. A FinFETstructure is preferable because it can suppress a short channel effectwhich occurs along with reduction in transistor size.

The above is the description of the structures and the manufacturingmethods of transistors. In this embodiment mode, a wiring, an electrode,a conductive layer, a conductive film, a terminal, a via, a plug, andthe like are preferably formed of one or more elements selected fromaluminum (Al), tantalum (Ta), titanium (Ti), molybdenum (Mo), tungsten(W), neodymium (Nd), chromium (Cr), nickel (Ni), platinum (Pt), gold(Au), silver (Ag), copper (Cu), magnesium (Mg), scandium (Sc), cobalt(Co), zinc (Zn), niobium (Nb), silicon (Si), phosphorus (P), boron (B),arsenic (As), gallium (Ga), indium (In), tin (Sn), and oxygen (O); or acompound or an alloy material including one or more of theaforementioned elements (e.g., indium tin oxide (ITO), indium zinc oxide(IZO), indium tin oxide containing silicon oxide (ITSO), zinc oxide(ZnO), tin oxide (SnO), cadmium tin oxide (CTO), aluminum neodymium(Al—Nd), magnesium silver (Mg—Ag), or molybdenum-niobium (Mo—Nb)); asubstance in which these compounds are combined; or the like.Alternatively, they are preferably formed to contain a substanceincluding a compound (silicide) of silicon and one or more of theaforementioned elements (e.g., aluminum silicon, molybdenum silicon, ornickel silicide); or a compound of nitrogen and one or more of theaforementioned elements (e.g., titanium nitride, tantalum nitride, ormolybdenum nitride).

Note that silicon (Si) may contain an n-type impurity (such asphosphorus) or a p-type impurity (such as boron). When silicon containsthe impurity, the conductivity is increased, and a function similar to ageneral conductor can be realized. Accordingly, such silicon can beutilized easily as a wiring, an electrode, or the like.

In addition, silicon with various levels of crystallinity, such assingle crystalline silicon, polycrystalline silicon, or microcrystallinesilicon can be used. Alternatively, silicon having no crystallinity,such as amorphous silicon can be used. By using single crystallinesilicon or polycrystalline silicon, resistance of a wiring, anelectrode, a conductive layer, a conductive film, a terminal, or thelike can be reduced. By using amorphous silicon or microcrystallinesilicon, a wiring or the like can be formed by a simple process.

Aluminum and silver have high conductivity, and thus can reduce signaldelay. Moreover, since aluminum and silver can be easily etched, theyare easily patterned and can be minutely processed.

Copper has high conductivity, and thus can reduce signal delay. Whencopper is used, a stacked-layer structure is preferably employed toimprove adhesion.

Molybdenum and titanium are preferable because even if molybdenum ortitanium is in contact with an oxide semiconductor (e.g., ITO or IZO) orsilicon, molybdenum or titanium does not cause defects. Moreover,molybdenum and titanium are preferable because they are easily etchedand has high heat resistance.

Tungsten is preferable because it has advantages such as high heatresistance.

Neodymium is also preferable because it has advantages such as high heatresistance. In particular, an alloy of neodymium and aluminum ispreferable because heat resistance is increased and aluminum hardlycauses hillocks.

Silicon is preferably used because it can be formed at the same time asa semiconductor layer included in a transistor and has high heatresistance.

Since ITO, IZO, ITSO, zinc oxide (ZnO), silicon (Si), tin oxide (SnO),and cadmium tin oxide (CTO) have light-transmitting properties, they canbe used for a portion which transmits light. For example, they can beused for a pixel electrode or a common electrode.

IZO is preferable because it is easily etched and processed. In etchingIZO, a residue is hardly left. Accordingly, when IZO is used for a pixelelectrode, defects (such as short circuit or orientation disorder) of aliquid crystal element or a light-emitting element can be reduced.

A wiring, an electrode, a conductive layer, a conductive film, aterminal, a via, a plug, or the like may have a single-layer structureor a multi-layer structure. By employing a single-layer structure, eachmanufacturing process of a wiring, an electrode, a conductive layer, aconductive film, a terminal, or the like can be simplified, the numberof days for a process can be reduced, and cost can be reduced.Alternatively, by employing a multi-layer structure, a wiring, anelectrode, and the like with high quality can be formed while anadvantage of each material is utilized and a disadvantage thereof isreduced. For example, when a low-resistant material (e.g., aluminum) isincluded in a multi-layer structure, reduction in resistance of a wiringcan be realized. As another example, when a stacked-layer structure inwhich a low heat-resistant material is interposed between highheat-resistant materials is employed, heat resistance of a wiring, anelectrode, and the like can be increased, utilizing advantages of thelow heat-resistance material. For example, it is preferable to employ astacked-layer structure in which a layer containing aluminum isinterposed between layers containing molybdenum, titanium, neodymium, orthe like.

When wirings, electrodes, or the like are in direct contact with eachother, they adversely affect each other in some cases. For example, onewiring or one electrode is mixed into a material of another wiring oranother electrode and changes its properties, and thus, an intendedfunction cannot be obtained in some cases. As another example, when ahigh-resistant portion is formed, a problem may occur so that it cannotbe normally formed. In such cases, a reactive material is preferablyinterposed by or covered with a non-reactive material in a stacked-layerstructure. For example, when ITO and aluminum are connected, titanium,molybdenum, or an alloy of neodymium is preferably interposed betweenITO and aluminum. As another example, when silicon and aluminum areconnected, titanium, molybdenum, or an alloy of neodymium is preferablyinterposed between silicon and aluminum.

The term “wiring” indicates a portion including a conductor. A wiringmay be a linear shape or made to be short without being extended.Therefore, an electrode is included in a wiring.

Note that a carbon nanotube may be used for a wiring, an electrode, aconductive layer, a conductive film, a terminal, a via, a plug, or thelike. Since a carbon nanotube has a light-transmitting property, it canbe used for a portion which transmits light. For example, a carbonnanotube can be used for a pixel electrode or a common electrode.

Note that although this embodiment mode is described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in each drawing in this embodiment modewith part of another embodiment mode.

Note that this embodiment mode shows examples of embodying, slightlytransforming, partially modifying, improving, describing in detail, orapplying the contents (or part of the contents) described in anotherembodiment mode, an example of related part thereof, or the like.Therefore, the contents described in another embodiment mode can befreely applied to, combined with, or replaced with this embodiment mode.

Embodiment Mode 7

In this embodiment mode, a structure of a display device is described.

A structure of a display device is described with reference to FIG. 38A.FIG. 38A is a top plan view of the display device.

A pixel portion 170101, a scan line input terminal 170103, and a signalline input terminal 170104 are formed over a substrate 170100. Scanlines extending in a row direction from the scan line input terminal170103 are formed over the substrate 170100, and signal lines extendingin a column direction from the signal line input terminal 170104 areformed over the substrate 170100. Pixels 170102 are arranged in matrixat each intersection of the scan lines and the signal lines in the pixelportion 170101.

The scan line side input terminal 170103 is formed on both sides of therow direction of the substrate 170100. Further, a scan line extendingfrom one scan line side input terminal 170103 and a scan line extendingfrom the other scan line side input terminal 170103 are alternatelyformed. In this case, since the pixels 170102 can be arranged in ahighly dense state, a high-definition display device can be obtained.However, the invention is not limited to this structure, and the scanline side input terminal 170103 may be formed only on one side of therow direction of the substrate 170100. In this case, a frame of thedisplay device can be made smaller. Moreover, the area of the pixelportion 170101 can be increased. As another example, the scan lineextending from one scan line side input terminal 170103 and the scanline extending from the other scan line side input terminal 170103 maybe used in common. In this case, the structure is suitable for displaydevices in which a load on a scan line is large, such as large-scaledisplay devices. Note that signals are input from an external drivercircuit to the scan line through the scan line side input terminal170103.

The signal line side input terminal 170104 is formed on one side of thecolumn direction of the substrate 170100. In this case, the frame of thedisplay device can be made smaller. Moreover, the area of the pixelportion 170101 can be increased. However, the invention is not limitedto this structure, and the signal line side input terminal 170104 may beformed on both sides of the column direction of the substrate 170100. Inthis case, the pixels 170102 are arranged with high density. Note thatsignals are input from an external driver circuit to the scan linethrough the signal line side input terminal 170104.

The pixel 170102 includes a switching element and a pixel electrode. Ineach pixel 170102, a first terminal of the switching element isconnected to the signal line, and a second terminal of the switchingelement is connected to the pixel electrode. On/off of the switchingelement is controlled by the scan line. However, the invention is notlimited to this structure, and a variety of structures can be employed.For example, the pixel 170102 may include a capacitor. In this case, acapacitor line is preferably formed over the substrate 170100. Asanother example, the pixel 170102 may include a current source such as adriving transistor. In this case, a power supply line is preferablyformed over the substrate 170100.

As the substrate 170100, a single crystalline substrate, an SOIsubstrate, a glass substrate, a quartz substrate, a plastic substrate, apaper substrate, a cellophane substrate, a stone substrate, a woodsubstrate, a cloth substrate (including a natural fiber (e.g., silk,cotton, or hemp), a synthetic fiber (e.g., nylon, polyurethane, orpolyester), and a regenerated fiber (e.g., acetate, cupra, rayon, orregenerated polyester)), a leather substrate, a rubber substrate, astainless steel substrate, a substrate including a stainless steel foil,or the like can be used. Alternatively, a skin (e.g., surfaces of theskin or corium) or hypodermal tissue of an animal such as a human can beused as the substrate. Note that the substrate 170100 is not limited tothose described above, and a variety of substrates can be used.

As the switching element included in the pixel 170102, a transistor(e.g., a bipolar transistor or a MOS transistor), a diode (e.g., a PNdiode, a PIN diode, a Schottky diode, an MIM (metal insulator metal)diode, an MIS (metal insulator semiconductor) diode, or adiode-connected transistor), a thyristor, or the like can be used. Notethat the switching element is not limited to those described above, anda variety of switching elements can be used. Note that when a MOStransistor is used as the switching element included in the pixel170102, a gate electrode is connected to the scan line, a first terminalis connected to the signal line, and a second terminal is connected tothe pixel electrode.

The above is the description of the case where a signal is input from anexternal driver circuit; however, the invention is not limited to thisstructure, and an IC chip can be mounted on a display device.

For example, as shown in FIG. 39A, an IC chip 170201 can be mounted onthe substrate 170100 by a COG (chip on glass) method. In this case, theIC chip 170201 can be examined before being mounted on the substrate170100, so that improvement in yield and reliability of the displaydevice can be realized. Note that portions common to those in FIG. 38Aare denoted by common reference numerals, and description thereof isomitted.

As another example, as shown in FIG. 39B, the IC chip 170201 can bemounted on an FPC (flexible printed circuit) 170200 by a TAB (tapeautomated bonding) method. In this case, the IC chip 170201 can beexamined before being mounted on the FPC 170200, so that improvement inyield and reliability of the display device can be realized. Note thatportions common to those in FIG. 38A are denoted by common referencenumerals, and description thereof is omitted.

Not only the IC chip can be mounted on the substrate 170100, but also adriver circuit can be formed over the substrate 170100.

For example, as shown in FIG. 38B, a scan line driver circuit 170105 canbe formed over the substrate 170100. In this case, the cost can bereduced by reduction in the number of components. Further, reliabilitycan be improved by reduction in the number of connection points betweencomponents. Since the driving frequency of the scan line driver circuit170105 is low, the scan line driver circuit 170105 can be easily formedusing amorphous silicon or microcrystalline silicon as a semiconductorlayer of a transistor. Note that an IC chip for outputting a signal tothe signal line may be mounted on the substrate 170100 by a COG method.Alternatively, an FPC on which an IC chip for outputting a signal to thesignal line is mounted by a TAB method may be provided on the substrate170100. In addition, an IC chip for controlling the scan line drivercircuit 170105 may be mounted on the substrate 170100 by a COG method.Alternatively, an FPC on which an IC chip for controlling the scan linedriver circuit 170105 is mounted by a TAB method may be provided on thesubstrate 170100. Note that portions common to those in FIG. 38A aredenoted by common reference numerals, and description thereof isomitted.

As another example, as shown in FIG. 38C, the scan line driver circuit170105 and a signal line driver circuit 170106 can be formed over thesubstrate 170100. Thus, the cost can be reduced by reduction in thenumber of components. Further, reliability can be improved by reductionin the number of connection points between components. Note that an ICchip for controlling the scan line driver circuit 170105 may be mountedon the substrate 170100 by a COG method. Alternatively, an FPC on whichan IC chip for controlling the scan line driver circuit 170105 ismounted by a TAB method may be provided on the substrate 170100. Inaddition, an IC chip for controlling the signal line driver circuit170106 may be mounted on the substrate 170100 by a COG method.Alternatively, an FPC on which an IC chip for controlling the signalline driver circuit 170106 is mounted by a TAB method may be provided onthe substrate 170100. Note that portions common to those in FIG. 38A aredenoted by common reference numerals, and description thereof isomitted.

Next, another structure of a display device is described with referenceto FIG. 40. Specifically, the display device includes a TFT substrate,an opposite substrate, and a display layer interposed between the TFTsubstrate and the opposite substrate. FIG. 40 is a top view of thedisplay device.

A pixel portion 170301, a scan line driver circuit 170302 a, a scan linedriver circuit 170302 b, and a signal line driver circuit 170303 areformed over a substrate 170300. The scan line driver circuits 170302 aand 170302 b and the signal line driver circuit 170303 are sealedbetween the substrate 170300 and a substrate 170310 with the use of asealant 170312.

Further, an FPC 107320 is arranged over the substrate 170300. Moreover,an IC chip 170321 is mounted on the FPC 170320 by a TAB method.

A plurality of pixels are arranged in matrix in the pixel portion170301. A scan line extending in the row direction from the scan linedriver circuit 170302 a is formed over the substrate 170300. A signalline extending in the column direction from the signal line drivercircuit 170303 is formed over the substrate 170300.

The scan line side input circuit 170302 a is formed on one side of therow direction of the substrate 170300. The scan line driver circuit170302 b is formed on the other side of the row direction of thesubstrate 170300. Further, a scan line extending from the scan line sideinput circuit 170302 a and a scan line extending from the scan line sideinput circuit 170302 b are alternately formed. Accordingly, ahigh-definition display device can be obtained. Note that the inventionis not limited to this structure, and only one of the scan line sideinput circuits 170302 a and 170302 b may be formed over the substrate170300. In this case, the frame of the display device can be madesmaller. Moreover, the area of the pixel portion 170301 can beincreased. As another example, the scan line extending from the scanline side input circuit 170302 a and the scan line extending from thescan line driver circuit 170302 b may be used in common. In this case,the structure is suitable for display devices in which a load on a scanline is large, such as large-scale display devices.

The signal line driver circuit 170303 is formed on one side of thecolumn direction of the substrate 170300. Accordingly, the frame of thedisplay device can be made smaller. The area of the pixel portion 170301can be increased. Note that the invention is not limited to thisstructure, and the signal line driver circuit 170303 may be formed onboth sides of the column direction of the substrate 170300. In thiscase, a high-definition display device can be obtained.

As the substrate 170300, a single crystalline substrate, an SOIsubstrate, a glass substrate, a quartz substrate, a plastic substrate, apaper substrate, a cellophane substrate, a stone substrate, a woodsubstrate, a cloth substrate (including a natural fiber (e.g., silk,cotton, or hemp), a synthetic fiber (e.g., nylon, polyurethane, orpolyester), and a regenerated fiber (e.g., acetate, cupra, rayon, orregenerated polyester)), a leather substrate, a rubber substrate, astainless steel substrate, a substrate including a stainless steel foil,or the like can be used. Alternatively, a skin (e.g., surfaces of theskin or corium) or hypodermal tissue of an animal such as a human can beused as the substrate. Note that the substrate 170300 is not limited tothose described above, and a variety of substrates can be used.

As the switching element included in the display device, a transistor(e.g., a bipolar transistor or a MOS transistor), a diode (e.g., a PNdiode, a PIN diode, a Schottky diode, an MIM (metal insulator metal)diode, an MIS (metal insulator semiconductor) diode, or adiode-connected transistor), a thyristor, or the like can be used. Notethat the switching element is not limited to those described above, anda variety of switching elements can be used.

The case where a driver circuit and a pixel portion are formed over thesame substrate has been described so far. Note that the invention is notlimited to this case, and another substrate over which the drivercircuit is partially or entirely formed may be made to be an IC chip sothat the substrate is mounted on the substrate over which the pixelportion is formed.

For example, as shown in FIG. 41A, an IC chip 170401 instead of thesignal line driver circuit can be mounted on the substrate 170300 by aCOG method. In this case, increase of power consumption can be preventedby mounting of the IC chip 170401 instead of the signal line drivercircuit on the substrate 170300 by a COG method. This is because thedrive frequency of the signal line driver circuit is high and thus powerconsumption is increased. Since the IC chip 170401 can be examinedbefore it is mounted on the substrate 170300, yield of a display devicecan be improved. moreover, reliability can be improved. Since the drivefrequency of the scan line driver circuits 170302 a and 170302 b is low,the scan line driver circuits 170302 a and 170302 b can be easily formedusing amorphous silicon or microcrystalline silicon for a semiconductorlayer of a transistor. Accordingly, a display device can be formed usinga large substrate. Note that portions which are common to those in thestructure of FIG. 40 are denoted by common reference numerals, and thedescription thereof is omitted.

As another example, as shown in FIG. 41B, the IC chip 170401 instead ofthe signal line driver circuit may be mounted on the substrate 170300 bya COG method, an IC chip 170501 a instead of the scan line drivercircuit 170302 a may be mounted on the substrate 170300 by a COG method,and an IC chip 170501 b instead of the scan line driver circuit 170302 bmay be mounted on the substrate 170300 by a COG method. In this case,since the IC chips 170401, 170501 a, and 170501 b can be examined beforethey are mounted on the substrate 170300, yield of a display device canbe improved. Moreover, reliability can be improved. It is easy to useamorphous silicon or microcrystalline silicon for a semiconductor layerof a transistor to be formed over the substrate 170300. Accordingly, adisplay device can be formed using a large substrate. Note that portionswhich are common to those in the structure of FIG. 40 are denoted bycommon reference numerals, and the description thereof is omitted.

Although this embodiment mode is described with reference to variousdrawings, the contents (or part of the contents) described in eachdrawing can be freely applied to, combined with, or replaced with thecontents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in each drawing in this embodiment modewith part of another embodiment mode.

This embodiment mode shows examples of embodying, slightly transforming,partially modifying, improving, describing in detailed, or applying thecontents (or part of the contents) described in another embodiment mode,an example of related part thereof, or the like. Therefore, the contentsdescribed in another embodiment mode can be freely applied to, combinedwith, or replaced with this embodiment mode.

Embodiment Mode 8

In this embodiment mode, an operation of a display device is described.

FIG. 42 shows a structure example of a display device.

A display device 180100 includes a pixel portion 180101, a signal linedriver circuit 180103, and a scan line driver circuit 180104. In thepixel portion 180101, a plurality of signal lines S1 to Sn extend fromthe signal line driver circuit 180103 in a column direction. In thepixel portion 180101, a plurality of scan lines G1 to Gm extend from thescan line driver circuit 180104 in a row direction. Pixels 180102 arearranged in matrix at each intersection of the plurality of signal linesSi to Sn and the plurality of scan lines G1 to Gm.

The signal line driver circuit 180103 has a function of outputting asignal to each of the signal lines S1 to Sn. This signal may be referredto as a video signal. The scan line driver circuit 180104 has a functionof outputting a signal to each of the scan lines G1 to Gm. This signalmay be referred to as a scan signal.

The pixel 180102 includes at least a switching element connected to thesignal line. On/off of the switching element is controlled by apotential of the scan line (a scan signal). When the switching elementis turned on, the pixel 180102 is selected. On the other hand, when theswitching element is turned off, the pixel 180102 is not selected.

When the pixel 180102 is selected (a selection state), a video signal isinput to the pixel 180102 from the signal line. A state (e.g.,luminance, transmittance, or voltage of a storage capacitor) of thepixel 180102 is changed in accordance with the video signal inputthereto.

When the pixel 180102 is not selected (a non-selection state), the videosignal is not input to the pixel 180102. Note that the pixel 180102holds a potential corresponding to the video signal which is input whenselected; thus, the pixel 180102 maintains the state (e.g., luminance,transmittance, or voltage of a storage capacitor) in accordance with thevideo signal.

Note that a structure of the display device is not limited to that shownin FIG. 42. For example, an additional wiring (such as a scan line, asignal line, a power supply line, a capacitor line, or a common line)may be added in accordance with the structure of the pixel 180102. Asanother example, a circuit having various functions may be added.

FIG. 43 shows an example of a timing chart for describing an operationof a display device.

The timing chart of FIG. 43 shows one frame period corresponding to aperiod when an image of one screen is displayed. One frame period is notparticularly limited, but is preferably 1/60 second or less so that aviewer does not perceive a flicker.

The timing chart of FIG. 43 shows timing of selecting the scan line G1in the first row, the scan line G1 (one of the scan lines G1 to Gm) inthe i-th row, the scan line Gi+1 in the (i+1)th row, and the scan lineGm in the m-th row.

At the same time as the scan line is selected, the pixel 180102connected to the scan line is also selected. For example, when the scanline Gi in the i-th row is selected, the pixel 180102 connected to thescan line Gi in the i-th row is also selected.

The scan lines G1 to Gm are sequentially selected (hereinafter alsoreferred to as scanned) from the scan line G1 in the first row to thescan line Gm in the m-th row. For example, while the scan line Gi in thei-th row is selected, the scan lines (G1 to Gi−1 and Gi+1 to Gm) otherthan the scan line Gi in the i-th row are not selected. Then, during thenext period, the scan line Gi+1 in the (i+1)th row is selected. Notethat a period during which one scan line is selected is referred to asone gate selection period.

Accordingly, when a scan line in a certain row is selected, videosignals from the signal lines S1 to Sn are input to a plurality ofpixels 180102 connected to the scan line, respectively. For example,while the scan line Gi in the i-th row is selected, given video signalsare input from the signal lines S1 to Sn to the plurality of pixels180102 connected to the scan line Gi in the i-th row, respectively.Thus, each of the plurality of pixels 180102 can be controlledindividually by the scan signal and the video signal.

Next, the case where one gate selection period is divided into aplurality of subgate selection periods is described. FIG. 44 is a timingchart in the case where one gate selection period is divided into twosubgate selection periods (a first subgate selection period and a secondsubgate selection period).

Note that one gate selection period may be divided into three or moresubgate selection periods.

The timing chart of FIG. 44 shows one frame period corresponding to aperiod when an image of one screen is displayed. One frame period is notparticularly limited, but is preferably 1/60 second or less so that aviewer does not perceive a flicker.

Note that one frame is divided into two subframes (a first subframe anda second subframe).

The timing chart of FIG. 44 shows timing of selecting the scan line Giin the i-th row, the scan line Gi+1 in the (i+1)th row, the scan line Gj(one of the scan lines Gi+1 to Gm) in the j-th row, and the scan lineGj+1 (one of the scan lines Gi+1 to Gm) in the (j+1)th row.

At the same time as the scan line is selected, the pixel 180102connected to the scan line is also selected. For example, when the scanline Gi in the i-th row is selected, the pixel 180102 connected to thescan line Gi in the i-th row is also selected.

The scan lines G1 to Gm are sequentially scanned in each subgateselection period. For example, in one gate selection period, the scanline Gi in the i-th row is selected in the first subgate selectionperiod, and the scan line Gj in the j-th row is selected in the secondsubgate selection period. Thus, in one gate selection period, anoperation can be performed as if the scan signals of two rows areselected. At this time, different video signals are input to the signallines S1 to Sn in the first subgate selection period and the secondsubgate selection period. Accordingly, different video signals can beinput to a plurality of pixels 180102 connected to the i-th row and aplurality of pixels 180102 connected to the j-th row.

Note that although this embodiment mode is described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in each drawing in this embodiment modewith part of another embodiment mode.

Note that this embodiment mode shows examples of embodying, slightlytransforming, partially modifying, improving, describing in detail, orapplying the contents (or part of the contents) described in anotherembodiment mode, an example of related part thereof, or the like.Therefore, the contents described in another embodiment mode can befreely applied to, combined with, or replaced with this embodiment mode.

Embodiment Mode 9

In this embodiment mode, a peripheral portion of a liquid crystal panelis described.

FIG. 45 shows an example of a liquid crystal display device including aso-called edge-light type backlight unit 20101 and a liquid crystalpanel 20107. An edge-light type corresponds to a type in which a lightsource is provided at an end of a backlight unit and fluorescence of thelight source is emitted from the entire light-emitting surface. Theedge-light type backlight unit is thin and can save power.

The backlight unit 20101 includes a diffusion plate 20102, a light guideplate 20103, a reflection plate 20104, a lamp reflector 20105, and alight source 20106.

The light source 20106 has a function of emitting light as necessary.For example, as the light source 20106, a cold cathode fluorescent lamp,a hot cathode fluorescent lamp, a light-emitting diode, an inorganic ELelement, an organic EL element, or the like is used. The lamp reflector20105 has a function of efficiently guiding fluorescence from the lightsource 20106 to the light guide plate 20103. The light guide plate 20103has a function of guiding light to the entire surface by totalreflection of the fluorescence. The diffusion plate 20102 has a functionof reducing variations in brightness. The reflection plate 20104 has afunction of reflecting light which is leaked from the light guide plate20103 downward (a direction which is opposite to the liquid crystalpanel 20107) to be reused.

Note that a control circuit for controlling luminance of the lightsource 20106 is connected to the backlight unit 20101. By using thiscontrol circuit, the luminance of the light source 20106 can becontrolled.

FIGS. 46A to 46D are views each showing a detailed structure of theedge-light type backlight unit. Note that description of a diffusionplate, a light guide plate, a reflection plate, and the like is omitted.

A backlight unit 20201 shown in FIG. 46A has a structure in which a coldcathode fluorescent lamp 20203 is used as a light source. In addition, alamp reflector 20202 is provided to efficiently reflect light from thecold cathode fluorescent lamp 20203. Such a structure is often used fora large display device because luminance of light obtained from the coldcathode fluorescent lamp is high.

A backlight unit 20211 shown in FIG. 46B has a structure in whichlight-emitting diodes (LEDs) 20213 are used as light sources. Forexample, the light-emitting diodes (LEDs) 20213 which emit white lightare provided at a predetermined interval. In addition, a lamp reflector20212 is provided to efficiently reflect light from the light-emittingdiodes (LEDs) 20213.

Since luminance of light-emitting diodes is high, a structure usinglight-emitting diodes is suitable for a large display device. Sincelight-emitting diodes are superior in color reproductivity, an imagewhich is closer to the real object can be displayed. Since chips of thelight-emitting diodes are small, the layout area can be reduced.Accordingly, a frame of a display device can be narrowed.

Note that in the case where light-emitting diodes are mounted on a largedisplay device, the light-emitting diodes can be provided on a back sideof the substrate. The light-emitting diodes of R, G, and B aresequentially provided at a predetermined interval. By providing thelight-emitting diodes, color reproductivity can be improved.

A backlight unit 20221 shown in FIG. 46C has a structure in whichlight-emitting diodes (LEDs) 20223, light-emitting diodes (LEDs) 20224,and light-emitting diodes (LEDs) 20225 of R, G, and B are used as lightsources. The light-emitting diodes (LEDs) 20223, the light-emittingdiodes (LEDs) 20224, and the light-emitting diodes (LEDs) 20225 of R, G,and B are each provided at a predetermined interval. By using thelight-emitting diodes (LEDs) 20223, the light-emitting diodes (LEDs)20224, and the light-emitting diodes (LEDs) 20225 of R, G, and B, colorreproductivity can be improved. In addition, a lamp reflector 20222 isprovided to efficiently reflect light from the light-emitting diodes.

Since luminance of light-emitting diodes is high, a structure usinglight-emitting diodes is suitable for a large display device. Sincelight-emitting diodes are superior in color reproductivity, an imagewhich is closer to the real object can be displayed. Since chips of theLEDs are small, the layout area can be reduced. Accordingly, a frame ofa display device can be narrowed.

By sequentially making the light-emitting diodes of R, G and B emitlight in accordance with time, color display can be performed. This is aso-called field sequential mode.

Note that a light-emitting diode which emits white light can be combinedwith the light-emitting diodes (LEDs) 20223, the light-emitting diodes(LEDs) 20224, and the light-emitting diodes (LEDs) 20225 of R, G and B.

Note that in the case where light-emitting diodes are mounted on a largedisplay device, the light-emitting diodes can be provided on a back sideof the substrate. The light-emitting diodes of R, G and B aresequentially provided at a predetermined interval. By providing thelight-emitting diodes, color reproductivity can be improved.

A backlight unit 20231 shown in FIG. 46D has a structure in whichlight-emitting diodes (LEDs) 20233, light-emitting diodes (LEDs) 20234,and light-emitting diodes (LEDs) 20235 of R, G and B are used as lightsources. For example, among the light-emitting diodes (LEDs) 20233, thelight-emitting diodes (LEDs) 20234, and the light-emitting diodes (LEDs)20235 of R, Q and B, a plurality of the light-emitting diodes of a colorwith low emission intensity (e.g., green) are provided. By using thelight-emitting diodes (LEDs) 20233, the light-emitting diodes (LEDs)20234, and the light-emitting diodes (LEDs) 20235 of R, Q and B, colorreproductivity can be improved. In addition, a lamp reflector 20232 isprovided to efficiently reflect light from the light-emitting diodes.

Since luminance of light-emitting diodes is high, a structure usinglight-emitting diodes is suitable for a large display device. Sincelight-emitting diodes are superior in color reproductivity, an imagewhich is closer to the real object can be displayed. Since chips of thelight-emitting diodes are small, the layout area can be reduced.Accordingly, a frame of a display device can be narrowed.

By sequentially making the light-emitting diodes of R, G, and B emitlight in accordance with time, color display can be performed. This is aso-called field sequential mode.

Note that a light-emitting diode which emits white light can be combinedwith the light-emitting diodes (LEDs) 20233, the light-emitting diodes(LEDs) 20234, and the light-emitting diodes (LEDs) 20235 of R, G, and B.

Note that in the case where light-emitting diodes are mounted on a largedisplay device, the light-emitting diodes can be provided on a back sideof the substrate. The light-emitting diodes of R, G, and B aresequentially provided at a predetermined interval. By providing thelight-emitting diodes, color reproductivity can be improved.

FIG. 49A shows an example of a liquid crystal display device including aso-called direct-type backlight unit and a liquid crystal panel. Adirect type corresponds to a type in which a light source is provideddirectly under a light-emitting surface and fluorescence of the lightsource is emitted from the entire light-emitting surface. Thedirect-type backlight unit can efficiently utilize the amount of emittedlight.

A backlight unit 20500 includes a diffusion plate 20501, alight-shielding plate 20502, a lamp reflector 20503, and a light source20504.

Light emitted from the light source 20504 is collected on one ofsurfaces of the backlight unit 20500 by the lamp reflector 20503. Thatis, the backlight unit 20500 has a surface through which light isstrongly emitted and a surface through which less light is emitted. Atthis time, when the liquid crystal panel 20505 is provided on the sideof the surface of the backlight unit 20500, through which light isstrongly emitted, the light emitted from the light source 20504 can beefficiently emitted to a liquid crystal panel 20505.

The light source 20504 has a function of emitting light as necessary.For example, as the light source 20504, a cold cathode fluorescent lamp,a hot cathode fluorescent lamp, a light-emitting diode, an inorganic ELelement, an organic EL element, or the like is used. The lamp reflector20503 has a function of efficiently guiding fluorescence from the lightsource 20504 to the diffusion plate 20501 and the light-shielding plate20502. The light-shielding plate 20502 has a function of reducingvariations in luminance by shielding much light as light becomes moreintense in accordance with provision of the light source 20504. Thediffusion plate 20501 also has a function of reducing variations inluminance.

A control circuit for controlling luminance of the light source 20504 isconnected to the backlight unit 20500. By using this control circuit,the luminance of the light source 20504 can be controlled.

FIG. 49B shows an example of a liquid crystal display device including aso-called direct-type backlight unit and a liquid crystal panel. Adirect type corresponds to a type in which a light source is provideddirectly under a light-emitting surface and fluorescence of the lightsource is emitted from the entire light-emitting surface. Thedirect-type backlight unit can efficiently utilize the amount of emittedlight.

A backlight unit 20510 includes a diffusion plate 20511; alight-shielding plate 20512; a lamp reflector 20513; and a light source(R) 20514 a, a light source (G) 20514 b, and a light source (B) 20514 cof R, G; and B.

Light emitted from each of the light source (R) 20514 a, the lightsource (G) 20514 b, and the light source (B) 20514 c is collected on oneof surfaces of the backlight unit 20510 by the lamp reflector 20513.That is, the backlight unit 20510 has a surface through which light isstrongly emitted and a surface through which less light is emitted. Atthis time, when a liquid crystal display panel 20515 is provided on thesurface side of the backlight unit 20510, through which light isstrongly emitted, the light emitted from each of the light source (R)20514 a, the light source (G) 20514 b, and the light source (B) 20514 ccan be sufficiently emitted to the liquid crystal display panel 20515.

Each of the light source (R) 20514 a, the light source (G) 20514 b, andthe light source (B) 20514 c of R, G, and B has a function of emittinglight as necessary. For example, as each of the light source (R) 20514a, the light source (G) 20514 b, and the light source (B) 20514 c, acold cathode fluorescent lamp, a hot cathode fluorescent lamp, alight-emitting diode, an inorganic EL element, an organic EL element, orthe like is used. The lamp reflector 20513 has a function of efficientlyguiding fluorescence from the light sources 20514 a to 20514 c to thediffusion plate 20511 and the light-shielding plate 20512. Thelight-shielding plate 20512 has a function of reducing variations inbrightness or luminance by shielding much light as light becomes moreintense in accordance with provision of the light sources 20514 a to20514 c. The diffusion plate 20511 also has a function of reducingvariations in brightness or luminance.

A control circuit for controlling luminance of the light source (R)20514 a, the light source (G) 20514 b, and the light source (B) 20514 cof R, G, and B is connected to the backlight unit 20510. By using thiscontrol circuit, the luminance of the light source (R) 20514 a, thelight source (G) 20514 b, and the light source (B) 20514 c of R, G, andB can be controlled.

FIG. 47 shows an example of a structure of a polarizing plate (alsoreferred to as a polarizing film).

A polarizing film 20300 includes a protective film 20301, a substratefilm 20302, a PVA polarizing film 20303, a substrate film 20304, anadhesive layer 20305, and a mold release film 20306.

The PVA polarizing film 20303 has a function of generating light in onlya certain vibration direction (linear polarized light). Specifically,the PVA polarizing film 20303 includes molecules (polarizers) in whichlengthwise electron density and widthwise electron density are greatlydifferent from each other. The PVA polarizing film 20303 can generatelinear polarized light by aligning directions of the molecules in whichlengthwise electron density and widthwise electron density are greatlydifferent from each other.

For example, a high molecular film of poly vinyl alcohol is doped withan iodine compound and a PVA film is pulled in a certain direction, sothat a film in which iodine molecules are aligned in a certain directioncan be obtained as the PVA polarizing film 20303. Then, light which isparallel to a major axis of the iodine molecule is absorbed by theiodine molecule. Note that a dichroic dye may be used instead of iodinefor high durability use and high heat resistance use. Note that it ispreferable that the dye be used for a liquid crystal display devicewhich needs to have durability and heat resistance, such as an in-carLCD or an LCD for a projector.

When the PVA polarizing film 20303 is sandwiched between films to serveas base materials (the substrate film 20302 and the substrate film20304), reliability can be improved. Note that the PVA polarizing film20303 may be sandwiched between triacetylcellulose (TAC) films with hightransparency and high durability. Note that each of the substrate filmsand the TAC films function as protective films of polarizer included inthe PVA polarizing film 20303.

The adhesive layer 20305 which is to be attached to a glass substrate ofthe liquid crystal panel is attached to one of the substrate films (thesubstrate film 20304). Note that the adhesive layer 20305 is formed byapplying an adhesive to one of the substrate films (the substrate film20304). The mold release film 20306 (a separate film) is provided forthe adhesive layer 20305.

The other of the substrates films (the substrate film 20302) is providedwith the protective film 20301.

A hard coating scattering layer (an anti-glare layer) may be provided ona surface of the polarizing film 20300. Since the surface of the hardcoating scattering layer has minute unevenness formed by AG treatmentand has an anti-glare function which scatters external light, reflectionof external light in the liquid crystal panel can be prevented. Surfacereflection can also be prevented.

Note that a treatment in which plurality of optical thin film layershaving different refractive indexes are layered (also referred to asanti-reflection treatment or AR treatment) may be performed on thesurface of the polarizing film 20300. The plurality of layered opticalthin film layers having different refractive indexes can reducereflectivity on the surface by an interference effect of light.

FIGS. 48A to 48C each show an example of a system block of the liquidcrystal display device.

In a pixel portion 20405, signal lines 20412 which are extended from asignal line driver circuit 20403 are arranged. Moreover, in the pixelportion 20405, scan lines 20410 which are extended from a scan linedriver circuit 20404 are also arranged. In addition, a plurality ofpixels are arranged in matrix in cross regions of the signal lines 20412and the scan lines 20410. Note that each of the plurality of pixelsincludes a switching element. Therefore, voltage for controllinginclination of liquid crystal molecules can be separately input to eachof the plurality of pixels. A structure in which a switching element isprovided in each cross region in this manner is referred to as an activetype. Note that the invention is not limited to such an active type anda structure of a passive type may be used. Since the passive type doesnot have a switching element in each pixel, a process is simple.

A driver circuit portion 20408 includes a control circuit 20402, thesignal line driver circuit 20403, and the scan line driver circuit20404. An image signal 20401 is input to the control circuit 20402. Thesignal line driver circuit 20403 and the scan line driver circuit 20404are controlled by the control circuit 20402 in accordance with thisimage signal 20401. That is, the control circuit 20402 inputs a controlsignal to each of the signal line driver circuit 20403 and the scan linedriver circuit 20404. Then, in accordance with this control signal, thesignal line driver circuit 20403 inputs a video signal to each of thesignal lines 20412 and the scan line driver circuit 20404 inputs a scansignal to each of the scan lines 20410. Then, the switching elementincluded in the pixel is selected in accordance with the scan signal andthe video signal is input to a pixel electrode of the pixel.

Note that the control circuit 20402 also controls a power source 20407in accordance with the image signal 20401. The power source 20407includes a unit for supplying power to a lighting unit 20406. As thelighting unit 20406, an edge-light type backlight unit or a direct-typebacklight unit can be used. Note that a front light may be used as thelighting unit 20406. A front light corresponds to a plate-like lightingunit including a luminous body and a light conducting body, which isattached to the front surface side of a pixel portion and illuminatesthe whole area. By using such a lighting unit, the pixel portion can beuniformly illuminated at low power consumption.

As shown in FIG. 48B, the scan line driver circuit 20404 includes ashift register 20441, a level shifter 20442, and a circuit functioningas a buffer 20443. A signal such as a gate start pulse (GSP) or a gateclock signal (GCK) is input to the shift register 20441.

As shown in FIG. 48C, the signal line driver circuit 20403 includes ashift register 20431, a first latch 20432, a second latch 20433, a levelshifter 20434, and a circuit functioning as a buffer 20435. The circuitfunctioning as the buffer 20435 corresponds to a circuit which has afunction of amplifying a weak signal and includes an operationalamplifier or the like. A signal such as a source start pulse (SSP) or asource clock signal (SCK) is input to the level shifter 20434, and data(DATA) such as a video signal is input to the first latch 20432. Latch(LAT) signals can be temporally held in the second latch 20433 and aresimultaneously input to the pixel portion 20405. This is referred to asline sequential driving. Therefore, when a pixel is used in which notline sequential driving but dot sequential driving is performed, thesecond latch can be omitted.

Note that in this embodiment mode, a known liquid crystal panel can beused for the liquid crystal panel. For example, a structure in which aliquid crystal layer is sealed between two substrates can be used as theliquid crystal panel. A transistor, a capacitor, a pixel electrode, analignment film, or the like is formed over one of the substrates. Apolarizing plate, a retardation plate, or a prism sheet may be providedon the surface opposite to a top surface of the one of the substrates. Acolor filter, a black matrix, a counter electrode, an alignment film, orthe like is provided on the other of the substrates. A polarizing plateor a retardation plate may be provided on the surface opposite to a topsurface of the other of the substrates. The color filter and the blackmatrix may be formed over the top surface of the one of the substrates.Note that three-dimensional display can be performed by providing a slit(a grid) on the top surface side of the one of the substrates or thesurface opposite to the top surface side of the one of the substrates.

Each of the polarizing plate, the retardation plate, and the prism sheetcan be provided between the two substrates. Alternatively, each of thepolarizing plate, the retardation plate, and the prism sheet can beintegrated with one of the two substrates.

Note that although this embodiment mode is described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, even more drawings can be formed by combining each part withanother part in the above-described drawings.

Similarly, the contents (or part of the contents) described in eachdrawing of this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, even more drawings can beformed by combining each part with part of another embodiment mode inthe drawings of this embodiment mode.

This embodiment mode shows an example of an embodied case of thecontents (or part of the contents) described in another embodiment mode,an example of slight transformation thereof, an example of partialmodification thereof, an example of improvement thereof, an example ofdetailed description thereof, an application example thereof, an exampleof related part thereof, or the like. Therefore, the contents describedin another embodiment mode can be freely applied to, combined with, orreplaced with this embodiment mode.

Embodiment Mode 10

In this embodiment mode, a pixel structure and an operation of a pixelwhich can be applied to a liquid crystal display device are described.

In this embodiment mode, as an operation mode of a liquid crystalelement, a TN (twisted nematic) mode, an IPS (in-plane-switching) mode,an FFS (fringe field switching) mode, an MVA (multi-domain verticalalignment) mode, a PVA (patterned vertical alignment) mode, an ASM(axially symmetric aligned micro-cell) mode, an OCB (optical compensatedbirefringence) mode, an FLC (ferroelectric liquid crystal) mode, an AFLC(antiferroelectric liquid crystal) mode, or the like can be used.

FIG. 50A shows an example of a pixel structure which can be applied tothe liquid crystal display device.

A pixel 40100 includes a transistor 40101, a liquid crystal element40102, and a capacitor 40103. A gate of the transistor 40101 isconnected to a wiring 40105. A first terminal of the transistor 40101 isconnected to a wiring 40104. A second terminal of the transistor 40101is connected to a first electrode of the liquid crystal element 40102and a first electrode of the capacitor 40103. A second electrode of theliquid crystal element 40102 corresponds to a counter electrode 40107. Asecond electrode of the capacitor 40103 is connected to a wiring 40106.

The wiring 40104 functions as a signal line. The wiring 40105 functionsas a scan line. The wiring 40106 functions as a capacitor line. Thetransistor 40101 functions as a switch. The capacitor 40103 functions asa storage capacitor.

It is acceptable as long as the transistor 40101 functions as a switch,and the transistor 40101 may be either a p-channel transistor or ann-channel transistor.

A video signal is input to the wiring 40104. A scan signal is input tothe wiring 40105. A constant potential is supplied to the wiring 40106.Note that the scan signal is an H-level or L-level digital voltagesignal. In the case where the transistor 40101 is an n-channeltransistor, an H level of the scan signal is a potential which can turnon the transistor 40101 and an L level of the scan signal is a potentialwhich can turn off the transistor 40101. Alternatively, in the casewhere the transistor 40101 is a p-channel transistor, the H level of thescan signal is a potential which can turn off the transistor 40101 andthe L level of the scan signal is a potential which can turn on thetransistor 40101. Note that the video signal is analog voltage. However,the invention is not limited thereto, and the video signal may bedigital voltage. Alternatively, the video signal may be current. Thecurrent of the video signal may be either analog current or digitalcurrent. The video signal has a potential which is lower than the Hlevel of the scan signal and higher than the L level of the scan signal.Note that the constant potential supplied to the wiring 40106 ispreferably equal to a potential of the counter electrode 40107.

Operations of the pixel 40100 are described by dividing the wholeoperations into the case where the transistor 40101 is on and the casewhere the transistor 40101 is off.

In the case where the transistor 40101 is on, the wiring 40104 iselectrically connected to the first electrode (a pixel electrode) of theliquid crystal element 40102 and the first electrode of the capacitor40103. Therefore, the video signal is input to the first electrode (thepixel electrode) of the liquid crystal element 40102 and the firstelectrode of the capacitor 40103 from the wiring 40104 through thetransistor 40101. In addition, the capacitor 40103 holds a potentialdifference between a potential of the video signal and the potentialsupplied to the wiring 40106.

In the case where the transistor 40101 is off, the wiring 40104 is notelectrically connected to the first electrode (the pixel electrode) ofthe liquid crystal element 40102 and the first electrode of thecapacitor 40103. Therefore, each of the first electrode of the liquidcrystal element 40102 and the first electrode of the capacitor 40103 isset in a floating state. Since the capacitor 40103 holds the potentialdifference between the potential of the video signal and the potentialsupplied to the wiring 40106, each of the first electrode of the liquidcrystal element 40102 and the first electrode of the capacitor 40103holds a potential which is the same as (corresponds to) the videosignal. Note that the liquid crystal element 40102 has transmissivity inaccordance with the video signal.

FIG. 50B shows an example of a pixel structure which can be applied tothe liquid crystal display device. In particular, FIG. 50B shows anexample of a pixel structure which can be applied to a liquid crystaldisplay device suitable for a horizontal electric field mode (includingan IPS mode and an FFS mode).

A pixel 40110 includes a transistor 40111, a liquid crystal element40112, and a capacitor 40113. A gate of the transistor 40111 isconnected to a wiring 40115. A first terminal of the transistor 40111 isconnected to a wiring 40114. A second terminal of the transistor 40111is connected to a first electrode of the liquid crystal element 40112and a first electrode of the capacitor 40113. A second electrode of theliquid crystal element 40112 is connected to a wiring 40116. A secondelectrode of the capacitor 40103 is connected to the wiring 40116.

The wiring 40114 functions as a signal line. The wiring 40115 functionsas a scan line. The wiring 40116 functions as a capacitor line. Thetransistor 40111 functions as a switch. The capacitor 40113 functions asa storage capacitor.

It is acceptable as long as the transistor 40111 functions as a switch,and the transistor 40111 may be a p-channel transistor or an n-channeltransistor.

A video signal is input to the wiring 40114. A scan signal is input tothe wiring 40115. A constant potential is supplied to the wiring 40116.Note that the scan signal is an H-level or L-level digital voltagesignal. In the case where the transistor 40111 is an n-channeltransistor, an H level of the scan signal is a potential which can turnon the transistor 40111 and an L level of the scan signal is a potentialwhich can turn off the transistor 40111. Alternatively, in the casewhere the transistor 40111 is a p-channel transistor, the H level of thescan signal is a potential which can turn off the transistor 40111 andthe L level of the scan signal is a potential which can turn on thetransistor 40111. Note that the video signal is analog voltage. However,the invention is not limited thereto, and the video signal may bedigital voltage. Alternatively, the video signal may be current. Thecurrent of the video signal may be either analog current or digitalcurrent. The video signal has a potential which is lower than the Hlevel of the scan signal and higher than the L level of the scan signal.

Operations of the pixel 40110 are described by dividing the wholeoperations into the case where the transistor 40111 is on and the casewhere the transistor 40111 is off.

In the case where the transistor 40111 is on, the wiring 40114 iselectrically connected to the first electrode (a pixel electrode) of theliquid crystal element 40112 and the first electrode of the capacitor40113. Therefore, the video signal is input to the first electrode (thepixel electrode) of the liquid crystal element 40112 and the firstelectrode of the capacitor 40113 from the wiring 40114 through thetransistor 40111. In addition, the capacitor 40113 holds a potentialdifference between a potential of the video signal and the potentialsupplied to the wiring 40116.

In the case where the transistor 40111 is off, the wiring 40114 is notelectrically connected to the first electrode (the pixel electrode) ofthe liquid crystal element 40112 and the first electrode of thecapacitor 40113. Therefore, each of the first electrode of the liquidcrystal element 40112 and the first electrode of the capacitor 40113 isset in a floating state. Since the capacitor 40113 holds the potentialdifference between the potential of the video signal and the potentialsupplied to the wiring 40116, each of the first electrode of the liquidcrystal element 40112 and the first electrode of the capacitor 40113holds a potential which is the same as (corresponds to) the videosignal. Note that the liquid crystal element 40112 has transmissivity inaccordance with the video signal.

FIG. 51 shows an example of a pixel structure which can be applied tothe liquid crystal display device. In particular, FIG. 51 shows anexample of a pixel structure in which an aperture ratio of a pixel canbe increased by reducing the number of wirings.

FIG. 51 shows two pixels which are provided in the same column direction(a pixel 40200 and a pixel 40210). For example, when the pixel 40200 isprovided in an N-th row, the pixel 40210 is provided in an (N+1)th row.

The pixel 40200 includes a transistor 40201, a liquid crystal element40202, and a capacitor 40203. A gate of the transistor 40201 isconnected to a wiring 40205. A first terminal of the transistor 40201 isconnected to a wiring 40204. A second terminal of the transistor 40201is connected to a first electrode of the liquid crystal element 40202and a first electrode of the capacitor 40203. A second electrode of theliquid crystal element 40202 corresponds to a counter electrode 40207. Asecond electrode of the capacitor 40203 is connected to a wiring whichis the same as a wiring connected to a gate of a transistor of theprevious row.

The pixel 40210 includes a transistor 40211, a liquid crystal element40212, and a capacitor 40213. A gate of the transistor 40211 isconnected to a wiring 40215. A first terminal of the transistor 40211 isconnected to the wiring 40204. A second terminal of the transistor 40211is connected to a first electrode of the liquid crystal element 40212and a first electrode of the capacitor 40213. A second electrode of theliquid crystal element 40212 corresponds to a counter electrode 40217. Asecond electrode of the capacitor 40213 is connected to the wiring whichis the same as the wiring connected to the gate of the transistor of theprevious row (the wiring 40205).

The wiring 40204 functions as a signal line. The wiring 40205 functionsas a scan line of the N-th row. The gate of the transistor of theprevious row functions as a capacitor line of the N-th row. Thetransistor 40201 functions as a switch. The capacitor 40203 functions asa storage capacitor.

A wiring 40214 functions as a signal line. The wiring 40215 functions asa scan line of the (N+1)th row. The wiring 40205 functions as acapacitor line of the (N+1)th row. The transistor 40211 functions as aswitch. The capacitor 40213 functions as a storage capacitor.

It is acceptable as long as each of the transistor 40201 and thetransistor 40211 functions as a switch, and each of the transistor 40201and the transistor 40211 may be either a p-channel transistor or ann-channel transistor.

A video signal is input to the wiring 40204. A scan signal (of the N-throw) is input to the wiring 40205. A scan signal (of the (N+1)th row) isinput to the wiring 40215.

The scan signal is an H-level or L-level digital voltage signal. In thecase where the transistor 40201 (or the transistor 40211) is ann-channel transistor, an H level of the scan signal is a potential whichcan turn on the transistor 40201 (or the transistor 40211) and an Llevel of the scan signal is a potential which can turn off thetransistor 40201 (or the transistor 40211). Alternatively, in the casewhere the transistor 40201 (or the transistor 40211) is a p-channeltransistor, the H level of the scan signal is a potential which can turnoff the transistor 40201 (or the transistor 40211) and the L level ofthe scan signal is a potential which can turn on the transistor 40201(or the transistor 40211). Note that the video signal is analog voltage.However, the invention is not limited thereto, and the video signal maybe digital voltage. Alternatively, the video signal may be current. Thecurrent of the video signal may be either analog current or digitalcurrent. The video signal has a potential which is lower than the Hlevel of the scan signal and higher than the L level of the scan signal.

Operations of the pixel 40200 are described by dividing the wholeoperations into the case where the transistor 40201 is on and the casewhere the transistor 40201 is off.

In the case where the transistor 40201 is on, the wiring 40204 iselectrically connected to the first electrode (a pixel electrode) of theliquid crystal element 40202 and the first electrode of the capacitor40203. Therefore, the video signal is input to the first electrode (thepixel electrode) of the liquid crystal element 40202 and the firstelectrode of the capacitor 40203 from the wiring 40204 through thetransistor 40201. In addition, the capacitor 40203 holds a potentialdifference between a potential of the video signal and a potentialsupplied to the wiring which is the same as the wiring connected to thegate of the transistor of the previous row.

In the case where the transistor 40201 is off, the wiring 40204 is notelectrically connected to the first electrode (the pixel electrode) ofthe liquid crystal element 40202 and the first electrode of thecapacitor 40203. Therefore, each of the first electrode of the liquidcrystal element 40202 and the first electrode of the capacitor 40203 isset in a floating state. Since the capacitor 40203 holds the potentialdifference between the potential of the video signal and the potentialof the wiring which is the same as the wiring connected to the gate ofthe transistor of the previous row, each of the first electrode of theliquid crystal element 40202 and the first electrode of the capacitor40203 holds a potential which is the same as (corresponds to) the videosignal. Note that the liquid crystal element 40202 has transmissivity inaccordance with the video signal.

Operations of the pixel 40210 are described by dividing the wholeoperations into the case where the transistor 40211 is on and the casewhere the transistor 40211 is off.

In the case where the transistor 40211 is on, the wiring 40214 iselectrically connected to the first electrode (a pixel electrode) of theliquid crystal element 40212 and the first electrode of the capacitor40213. Therefore, the video signal is input to the first electrode (thepixel electrode) of the liquid crystal element 40212 and the firstelectrode of the capacitor 40213 from the wiring 40214 through thetransistor 40211. In addition, the capacitor 40213 holds a potentialdifference between a potential of the video signal and a potentialsupplied to a wiring which is the same as the wiring connected to thegate of the transistor of the previous row (the wiring 40205).

In the case where the transistor 40211 is off, the wiring 40204 is notelectrically connected to the first electrode (the pixel electrode) ofthe liquid crystal element 40212 and the first electrode of thecapacitor 40213. Therefore, each of the first electrode of the liquidcrystal element 40212 and the first electrode of the capacitor 40213 isset in a floating state. Since the capacitor 40103 holds the potentialdifference between the potential of the video signal and the potentialof the wiring which is the same as the wiring connected to the gate ofthe transistor of the previous row (the wiring 40215), each of the firstelectrode (the pixel electrode) of the liquid crystal element 40212 andthe first electrode of the capacitor 40213 holds a potential which isthe same as (corresponds to) the video signal. Note that the liquidcrystal element 40212 has transmissivity in accordance with the videosignal.

FIG. 52 shows an example of a pixel structure which can be applied tothe liquid crystal display device. In particular, FIG. 52 shows anexample of a pixel structure in which a viewing angle can be improved byusing a subpixel.

A pixel 40320 includes a subpixel 40300 and a subpixel 40310. Althoughthe case in which the pixel 40320 includes two subpixels is described,the pixel 40320 may include three or more subpixels.

The subpixel 40300 includes a transistor 40301, a liquid crystal element40302, and a capacitor 40303. A gate of the transistor 40301 isconnected to a wiring 40305. A first terminal of the transistor 40301 isconnected to a wiring 40304. A second terminal of the transistor 40301is connected to a first electrode of the liquid crystal element 40302and a first electrode of the capacitor 40303. A second electrode of theliquid crystal element 40302 corresponds to a counter electrode 40307. Asecond electrode of the capacitor 40303 is connected to a wiring 40306.

The subpixel 40310 includes a transistor 40311, a liquid crystal element40312, and a capacitor 40313. A gate of the transistor 40311 isconnected to a wiring 40315. A first terminal of the transistor 40311 isconnected to the wiring 40304. A second terminal of the transistor 40311is connected to a first electrode of the liquid crystal element 40312and a first electrode of the capacitor 40313. A second electrode of theliquid crystal element 40312 corresponds to a counter electrode 40317. Asecond electrode of the capacitor 40313 is connected to the wiring40306.

The wiring 40304 functions as a signal line. The wiring 40305 functionsas a scan line. The wiring 40315 functions as a signal line. The wiring40306 functions as a capacitor line. Each of the transistor 40301 andthe transistor 40311 functions as a switch. Each of the capacitor 40303and the capacitor 40313 functions as a storage capacitor.

It is acceptable as long as each of the transistor 40301 and thetransistor 40311 functions as a switch, and each of the transistor 40301and the transistor 40311 may be either a p-channel transistor or ann-channel transistor.

A video signal is input to the wiring 40304. A scan signal is input tothe wiring 40305. A scan signal is input to the wiring 40315. A constantpotential is supplied to the wiring 40306.

The scan signal is an H-level or L-level digital voltage signal. In thecase where the transistor 40301 (or the transistor 40311) is ann-channel transistor, an H level of the scan signal is a potential whichcan turn on the transistor 40301 (or the transistor 40311) and an Llevel of the scan signal is a potential which can turn off thetransistor 40301 (or the transistor 40311). Alternatively, in the casewhere the transistor 40301 (or the transistor 40311) is a p-channeltransistor, the H level of the scan signal is a potential which can turnoff the transistor 40301 (or the transistor 40311) and the L level ofthe scan signal is a potential which can turn on the transistor 40301(or the transistor 40311). Note that the video signal is analog voltage.However, the invention is not limited thereto, and the video signal maybe digital voltage. Alternatively, the video signal may be current. Thecurrent of the video signal may be either analog current or digitalcurrent. The video signal has a potential which is lower than the Hlevel of the scan signal and higher than the L level of the scan signal.Note that the constant potential supplied to the wiring 40306 ispreferably equal to a potential of the counter electrode 40307 or apotential of the counter electrode 40307.

Operations of the pixel 40320 are described by dividing the wholeoperations into the case where the transistor 40301 is on and thetransistor 40311 is off, the case where the transistor 40301 is off andthe transistor 40311 is on, and the case where the transistor 40301 andthe transistor 40311 are off.

In the case where the transistor 40301 is on and the transistor 40311 isoff, the wiring 40304 is electrically connected to the first electrode(a pixel electrode) of the liquid crystal element 40302 and the firstelectrode of the capacitor 40303 in the subpixel 40300. Therefore, thevideo signal is input to the first electrode (the pixel electrode) ofthe liquid crystal element 40302 and the first electrode of thecapacitor 40303 from the wiring 40304 through the transistor 40301. Inaddition, the capacitor 40303 holds a potential difference between apotential of the video signal and a potential supplied to the wiring40306. At this time, the wiring 40304 is not electrically connected tothe first electrode (the pixel electrode) of the liquid crystal element40312 and the first electrode of the capacitor 40313 in the subpixel40310. Therefore, the video signal is not input to the subpixel 40310.

In the case where the transistor 40301 is off and the transistor 40311is on, the wiring 40304 is not electrically connected to the firstelectrode (the pixel electrode) of the liquid crystal element 40302 andthe first electrode of the capacitor 40303 in the subpixel 40300.Therefore, each of the first electrode of the liquid crystal element40302 and the first electrode of the capacitor 40303 is set in afloating state. Since the capacitor 40303 holds the potential differencebetween the potential of the video signal and the potential supplied tothe wiring 40306, each of the first electrode of the liquid crystalelement 40302 and the first electrode of the capacitor 40303 holds apotential which is the same as (corresponds to) the video signal. Atthis time, the wiring 40304 is electrically connected to the firstelectrode (the pixel electrode) of the liquid crystal element 40312 andthe first electrode of the capacitor 40313 in the subpixel 40310.Therefore, the video signal is input to the first electrode (the pixelelectrode) of the liquid crystal element 40312 and the first electrodeof the capacitor 40313 from the wiring 40304 through the transistor40311. In addition, the capacitor 40313 holds a potential differencebetween a potential of the video signal and a potential supplied to thewiring 40306.

In the case where the transistor 40301 and the transistor 40311 are off,the wiring 40304 is not electrically connected to the first electrode(the pixel electrode) of the liquid crystal element 40302 and the firstelectrode of the capacitor 40303 in the subpixel 40300. Therefore, eachof the first electrode of the liquid crystal element 40302 and the firstelectrode of the capacitor 40303 is set in a floating state. Since thecapacitor 40303 holds the potential difference between the potential ofthe video signal and the potential supplied to the wiring 40306, each ofthe first electrode of the liquid crystal element 40302 and the firstelectrode of the capacitor 40303 holds a potential which is the same as(corresponds to) the video signal. Note that the liquid crystal element40302 has transmissivity in accordance with the video signal. At thistime, the wiring 40304 is not electrically connected to the firstelectrode (the pixel electrode) of the liquid crystal element 40312 andthe first electrode of the capacitor 40313 similarly in the subpixel40310. Therefore, each of the first electrode of the liquid crystalelement 40312 and the first electrode of the capacitor 40313 is set in afloating state. Since the capacitor 40313 holds the potential differencebetween the potential of the video signal and the potential of thewiring 40316, each of the first electrode of the liquid crystal element40312 and the first electrode of the capacitor 40313 holds a potentialwhich is the same as (corresponds to) the video signal. Note that theliquid crystal element 40312 has transmissivity in accordance with thevideo signal.

A video signal input to the subpixel 40300 may be a value which isdifferent from that of a video signal input to the subpixel 40310. Inthis case, the viewing angle can be widened because alignment of liquidcrystal molecules of the liquid crystal element 40302 and alignment ofliquid crystal molecules of the liquid crystal element 40312 can bevaried from each other.

Although this embodiment mode is described with reference to variousdrawings, the contents (or part of the contents) described in eachdrawing can be freely applied to, combined with, or replaced with thecontents (or part of the contents) described in another drawing.Further, even more drawings can be formed by combining each part withanother part in the above-described drawings.

Similarly, the contents (or part of the contents) described in eachdrawing of this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, even more drawings can beformed by combining each part with part of another embodiment mode inthe drawings of this embodiment mode.

This embodiment mode shows an example of an embodied case of thecontents (or part of the contents) described in another embodiment mode,an example of slight transformation thereof, an example of partialmodification thereof, an example of improvement thereof, an example ofdetailed description thereof, an application example thereof, an exampleof related part thereof, or the like. Therefore, the contents describedin another embodiment mode can be freely applied to, combined with, orreplaced with this embodiment mode.

Embodiment Mode 11

In this embodiment mode, a driving method of a display device isdescribed. In particular, a driving method of a liquid crystal displaydevice is described.

A liquid crystal panel which can be used for the liquid crystal displaydevice described in this embodiment mode has a structure in which aliquid crystal material is interposed between two substrates. Each ofthe two substrates is provided with an electrode for controlling anelectric field applied to the liquid crystal material. A liquid crystalmaterial corresponds to a material whose optical and electricalproperties are changed by an electric field externally applied.Accordingly, a liquid crystal panel corresponds to a device in whichdesired optical and electrical properties can be obtained by controllingvoltage applied to the liquid crystal material with use of the electrodeprovided for each of the two substrates. Further, a plurality ofelectrodes are arranged in a planar manner so that each of theelectrodes corresponds to a pixel, and voltages applied to the pixelsare individually controlled; thus, a liquid crystal display panel whichcan display a high-definition image can be obtained.

Here, response time of the liquid crystal material due to change in anelectric field depends on a space (a cell gap) between the twosubstrates and a type or the like of the liquid crystal material, and isgenerally several milliseconds to several ten milliseconds. When theamount of change in the electric field is small, the response time ofthe liquid crystal material is further lengthened. This characteristiccauses defects in image display, such as an after image, a phenomenon inwhich traces are seen, and decrease in contrast when the liquid crystalpanel displays a moving image. In particular, when a half tone ischanged into another half tone (when change in the electric field issmall), a degree of the above-described defects become pronounced.

On the other hand, as a particular problem for a liquid crystal panelusing an active matrix method, fluctuation in writing voltage due toconstant charge driving is given. Constant charge driving in thisembodiment mode is described below.

A pixel circuit using an active matrix method includes a switch whichcontrols writing and a capacitor which holds a charge. A method fordriving the pixel circuit using the active matrix method corresponds toa method in which predetermined voltage is written in the pixel circuitwith a switch in an on state, and thereafter, the switch is immediatelyturned off and a charge in the pixel circuit is held (a hold state). Atthe time of the hold state, exchange of the charge between inside andoutside of the pixel circuit is not performed (a constant charge). Ingeneral, a period during which the switch is in an off state isapproximately several hundreds (the number of scan lines) of timeslonger than a period during which the switch is in an on state.Accordingly, it is likely that the switch of the pixel circuit is almostalways in an off state. As described above, constant charge driving inthis embodiment mode corresponds to a driving method in which a pixelcircuit is in a hold state in almost all periods when a liquid crystalpanel is driven.

Next, electrical properties of the liquid crystal material aredescribed. A dielectric constant as well as optical properties of theliquid crystal material are changed when an electric field externallyapplied is changed. That is, when it is considered that each pixel ofthe liquid crystal panel is a capacitor (a liquid crystal element)interposed between two electrodes, the capacitor corresponds to acapacitor whose capacitance is changed in accordance with appliedvoltage. This phenomenon is called dynamic capacitance.

When a capacitor whose capacitance is changed in accordance with appliedvoltage in this manner is driven by the constant charge driving, thefollowing problem occurs. When capacitance of a liquid crystal elementis changed in a hold state in which a charge is not moved, appliedvoltage is also changed. This can be understood from the fact that theamount of charges is constant in a relational expression of (the amountof charges)=(capacitance)×(applied voltage).

For the above-described reasons, voltage at the time of a hold state ischanged from voltage at the time of writing because constant chargedriving is performed in a liquid crystal panel using an active matrixmethod. Accordingly, change in transmittance of the liquid crystalelement is different from change in transmittance of a liquid crystalelement in a driving method which does not take a hold state. FIGS. 56Ato 56C illustrate this state. FIG. 56A shows an example of control ofvoltage written in a pixel circuit when time is represented by thehorizontal axis and an absolute value of the voltage is represented bythe vertical axis. FIG. 56B shows an example of control of voltagewritten in the pixel circuit when time is represented by the horizontalaxis and the voltage is represented by the vertical axis. FIG. 56C showschange in transmittance of the liquid crystal element over time in thecase where the voltage shown in FIG. 56A or 56B is written in the pixelcircuit when time is represented by the horizontal axis andtransmittance of the liquid crystal element is represented by thevertical axis. In each of FIGS. 56A to 56C, a period F indicates aperiod for rewriting the voltage, and time for rewriting the voltage isdenoted by t₁, t₂, t₃, t₄, and the like.

Here, writing voltage corresponding to image data input to the liquidcrystal display device corresponds to |V₁| in rewriting at the time of 0and corresponds to |V₂| in rewriting at the time of t₁, t₂, t₃, t₄, andthe like (see FIG. 56A).

Note that polarity of the writing voltage corresponding to image datainput to the liquid crystal display device may be switched periodically(inversion driving: see FIG. 56B). Since direct voltage can be preventedfrom being applied to a liquid crystal as much as possible by using thismethod, burn-in or the like caused by deterioration of the liquidcrystal element can be prevented. Note that a period of switching thepolarity (an inversion period) may be the same as a period of rewritingvoltage. In this case, generation of flickers caused by inversiondriving can be reduced because the inversion period is short. Further,the inversion period may be a period which is integral times the periodof rewriting voltage. In this case, power consumption can be reducedbecause the inversion period is long and frequency of writing voltagecan be decreased by changing the polarity.

FIG. 56C shows change in transmittance of the liquid crystal elementover time when voltage as shown in FIG. 56A or 56B is applied to theliquid crystal element. Here, transmittance of the liquid crystalelement in the case where enough time passes after the voltage |V₁| isapplied to the liquid crystal element corresponds to TR₁. Similarly,transmittance of the liquid crystal element in the case where enoughtime passes after the voltage |V₂| is applied to the liquid crystalelement corresponds to TR₂. When the voltage applied to the liquidcrystal element is changed from |V₁| to |V₂| at the time of t₁,transmittance of the liquid crystal element does not immediately becomeTR₂ but slowly changes as shown by a dashed line 30401. For example,when the period of rewriting voltage is the same as a frame period (16.7milliseconds) of an image signal of 60 Hz, time for several frames isneeded until transmittance is changed to TR₂.

Note that smooth change in transmittance over time as shown in thedashed line 30401 corresponds to change in transmittance over time whenthe voltage |V₂| is accurately applied to the liquid crystal element. Inan actual liquid crystal panel, for example, in a liquid crystal panelusing an active matrix method, transmittance of the liquid crystalelement does not changed over time as shown by the dashed line 30401 butgradually changes over time as shown by a solid line 30402. This isbecause voltage at the time of a hold state is changed due to constantcharge driving, and it is impossible to reach intended voltage only byone writing. Accordingly, the response time of transmittance of theliquid crystal element becomes further longer than original responsetime (the dashed line 30401) in appearance, so that defects when animage is displayed, such as an after image, a phenomenon in which tracesare seen, or decrease in contrast noticeably occur.

By using overdriving, it is possible to solve a phenomenon in which theresponse time in appearance becomes further longer due to shortage ofwriting by dynamic capacitance and constant charge driving as well aslength of the original response time of the liquid crystal element.FIGS. 57A to 57C illustrate this state. FIG. 57A shows an example ofcontrol of voltage written in a pixel circuit when time is representedby the horizontal axis and an absolute value of the voltage isrepresented by the vertical axis. FIG. 57B shows an example of controlof voltage written in the pixel circuit when time is represented by thehorizontal axis and the voltage is represented by the vertical axis.FIG. 57C shows change in transmittance of the liquid crystal elementover time in the case where the voltage shown in FIG. 57A or 57B iswritten in the pixel circuit when time is represented by the horizontalaxis and transmittance of the liquid crystal element is represented bythe vertical axis. In each of FIGS. 57A to 57C, a period F indicates aperiod for rewriting the voltage, and time for rewriting the voltage isdenoted by t₁, t₂, t₃, t₄, and the like.

Here, writing voltage corresponding to image data input to the liquidcrystal display device corresponds to |V₁| in rewriting at the time of0, corresponds to |V₃| in rewriting at the time of t₁, and correspondsto |V₂| in rewriting at the time of t₂, t₃, t₄, and the like (see FIG.57A).

Note that polarity of the writing voltage corresponding to image datainput to the liquid crystal display device may be switched periodically(inversion driving: see FIG. 57B). Since direct voltage can be preventedfrom being applied to a liquid crystal as much as possible by using thismethod, burn-in or the like caused by deterioration of the liquidcrystal element can be prevented. Note that a period of switching thepolarity (an inversion period) may be the same as a period of rewritingvoltage. In this case, generation of flickers caused by inversiondriving can be reduced because the inversion period is short. Further,the inversion period may be a period which is integral times the periodof rewriting voltage. In this case, power consumption can be reducedbecause the inversion period is long and frequency of writing voltagecan be decreased by changing the polarity.

FIG. 57C shows change in transmittance of the liquid crystal elementover time when voltage as shown in FIG. 57A or 57B is applied to theliquid crystal element. Here, transmittance of the liquid crystalelement in the case where enough time passes after the voltage |V₁| isapplied to the liquid crystal element corresponds to TR₁. Similarly,transmittance of the liquid crystal element in the case where enoughtime passes after the voltage |V₂| is applied to the liquid crystalelement corresponds to TR₂. Similarly, transmittance of the liquidcrystal element in the case where enough time passes after the voltage|V₃| is applied to the liquid crystal element corresponds to TR₃. Whenthe voltage applied to the liquid crystal element is changed from |V₁|to |V₃| at the time of t₁, transmittance of the liquid crystal elementis tried to be changed to TR₃ for several frames as shown by a dashedline 30501. However, application of the voltage |V₃| is terminated atthe time of t₂, and the voltage |V₂| is applied after the time of t₂.Accordingly, transmittance of the liquid crystal element does not becomeas shown by the dashed line 30501 but becomes as shown by a solid line30502. A value of the voltage |V₃| is preferably set so thattransmittance is approximately TR₂ at the time of t₂. Here, the voltage|V₃| is also referred to as overdriving voltage. That is, the responsetime of the liquid crystal element can be controlled to some extent bychanging |V₃|, which is the overdriving voltage. This is because theresponse time of the liquid crystal element is changed by the strengthof an electric field. Specifically, the response time of the liquidcrystal element becomes shorter as the electric field is stronger, andthe response time of the liquid crystal element becomes longer as theelectric field is weaker.

Note that |V₃|, which is the overdriving voltage, is preferably changedin accordance with the amount of change in the voltage, that is, thevoltage |V₁| and the voltage |V₂| which provide intended transmittanceTR₁ and TR₂. This is because appropriate response time can be alwaysobtained by changing |V₃|, which is the overdriving voltage, inaccordance with change in the response time of the liquid crystalelement even when the response time of the liquid crystal element ischanged by the amount of change in the voltage.

In addition, |V₃|, which is the overdriving voltage, is preferablychanged depending on a mode of the liquid crystal element, such as a TNmode, a VA mode, an IPS mode, or an OCB mode. This is becauseappropriate response time can be always obtained by changing |V₃|, whichis the overdriving voltage, in accordance with change in the responsetime of the liquid crystal element even when the response time of theliquid crystal element is changed depending on the mode of the liquidcrystal element.

Note that the voltage rewriting period F may be the same as a frameperiod of an input signal. In this case, since a peripheral drivercircuit of a liquid crystal display device can be simplified, the liquidcrystal display device with low manufacturing cost can be obtained.

Note that the voltage rewriting period F may be shorter than the frameperiod of the input signal. For example, the voltage rewriting period Fmay be half the frame period of the input signal, or one third or lessthe frame period of the input signal. It is effective to combine thismethod with a measure against deterioration in quality of a moving imagecaused by hold driving of the liquid crystal display device, such asblack data insertion driving, backlight blinking, backlight scanning, orintermediate image insertion driving by motion compensation. That is,since required response time of the liquid crystal element is short inthe measure against deterioration in quality of a moving image caused byhold driving of the liquid crystal display device, the response time ofthe liquid crystal element can be relatively shortened easily by usingthe overdriving method described in this embodiment mode. Although theresponse time of the liquid crystal element can be essentially shortenedby a cell gap, a liquid crystal material, a mode of the liquid crystalelement, or the like, it is technically difficult to shorten theresponse time of the liquid crystal element. Accordingly, it is veryimportant to use a method for shortening the response time of the liquidcrystal element by a driving method, such as overdriving.

Note also that the voltage rewriting period F may be longer than theframe period of the input signal. For example, the voltage rewritingperiod F may be twice the frame period of the input signal, or threetimes or more the frame period of the input signal. It is effective tocombine this method with a means (a circuit) which determines whethervoltage is not rewritten for a long period or not. That is, when thevoltage is not rewritten for a long period, an operation of the circuitcan be stopped during a period where no voltage is rewritten withoutperforming a rewriting operation of the voltage. Thus, a liquid crystaldisplay device with low power consumption can be obtained.

Next, a specific method for changing the overdriving voltage |V₃| inaccordance with the voltage |V₁| and the voltage |V₂|, which provideintended transmittance TR₁ and TR₂, is described.

Since an overdriving circuit is a circuit for controlling theoverdriving voltage |V₃| in accordance with the voltage |V₁| and thevoltage |V₂|, which provide intended transmittance TR₁ and TR₂, asappropriate, signals input to the overdriving circuit are a signalrelated to the voltage |V₁|, which provides intended transmittance TR₁,and a signal related to the voltage |V₂|, which provides intendedtransmittance TR₂. A signal output from the overdriving circuit is asignal related to the overdriving voltage |V₃|. Here, each of thesesignals may have an analog voltage value such as the voltage (|V₁|,|V₂|, or |V₃|) applied to the liquid crystal element or may be a digitalsignal for supplying the voltage applied to the liquid crystal element.Here, the signal related to the overdriving circuit is described as adigital signal.

First, a general structure of the overdriving circuit is described withreference to FIG. 53A. Here, input image signals 30101 a and 30101 b areused as signals for controlling the overdriving voltage. As a result ofprocessing these signals, an output image signal 30104 is to be outputas a signal which provides the overdriving voltage.

Since the voltage |V₁| and the voltage |V₂|, which provide intendedtransmittance TR₁ and TR₂, are image signals in adjacent frames, it ispreferable that the input image signals 30101 a and 30101 b be alsoimage signals in adjacent frames. In order to obtain such signals, theinput image signal 30101 a is input to a delay circuit 30102 in FIG.53A, and a signal which is consequently output can be used as the inputimage signal 30101 b. An example of the delay circuit 30102 includes amemory. That is, the input image signal 30101 a is stored in the memoryin order to delay the input image signal 30101 a for one frame, and atthe same time, a signal stored in the previous frame is extracted fromthe memory as the input image signal 30101 b, and the input imagesignals 30101 a and 30101 b are simultaneously input to a correctioncircuit 30103. Thus, the image signals in adjacent frames can behandled. By inputting the image signals in adjacent frames to thecorrection circuit 30103, the output image signal 30104 can be obtained.Note that when a memory is used as the delay circuit 30102, a memoryhaving capacity for storing an image signal for one frame in order todelay the input image signal 30101 a for one frame (i.e., a framememory) can be obtained. Thus, the memory can have a function as a delaycircuit without causing excess and deficiency of memory capacity.

Next, the delay circuit 30102 formed mainly for reducing memory capacityis described. Since memory capacity can be reduced using such a circuitas the delay circuit 30102, manufacturing cost can be reduced.

Specifically, a delay circuit as shown in FIG. 53B can be used as thedelay circuit 30102 having such characteristics. The delay circuit shownin FIG. 53B includes an encoder 30105, a memory 30106, and a decoder30107.

Operations of the delay circuit 30102 shown in FIG. 53B are as follows.First, compression processing is performed by the encoder 30105 beforethe input image signal 30101 a is stored in the memory 30106. Thus, thesize of data to be stored in the memory 30106 can be reduced.Accordingly, memory capacity can be reduced, and manufacturing cost canbe reduced. Then, the compressed image signal is transmitted to thedecoder 30107 in which extension processing is performed. Thus, thesignal which has been compressed by the encoder 30105 can be restored.Here, compression processing and extension processing performed by theencoder 30105 and the decoder 30107 may be reversible processing.Accordingly, since the image signal does not deteriorate even aftercompression processing and extension processing are performed, memorycapacity can be reduced without causing deterioration of quality of animage which is finally displayed on a device. Alternatively, compressionprocessing and extension processing performed by the encoder 30105 andthe decoder 30107 may be non-reversible processing. Accordingly, sincethe size of data of the compressed image signal can be made extremelysmall, memory capacity can be significantly reduced.

As a method for reducing memory capacity, a variety of methods can beused as well as the above-described method. For example, a method inwhich color information included in an image signal is reduced (e.g.,tone reduction from 260 thousand colors to 65 thousand colors isperformed) or the amount of data is reduced (resolution is reduced)without image compression by an encoder can be used.

Next, specific examples of the correction circuit 30103 are describedwith reference to FIGS. 53C to 53E. The correction circuit 30103 is acircuit for outputting an output image signal of a certain value fromtwo input image signals. Here, when a relation between the two inputimage signals and the output image signal is non-linear and it isdifficult to calculate the relation by simple operation, a look up table(LUT) may be used as the correction circuit 30103. Since the relationbetween the two input image signals and the output image signal iscalculated in advance by measurement in the LUT, the output image signalcorresponding to the two input image signals can be calculated only byseeing the LUT (see FIG. 53C). When a LUT 30108 is used as thecorrection circuit 30103, the correction circuit 30103 can be realizedwithout complicated circuit design or the like.

Since the LUT is one of memories, it is preferable to reduce memorycapacity as much as possible in order to reduce manufacturing cost. Asan example of the correction circuit 30103 for realizing reduction inmemory capacity, a circuit shown in FIG. 53D can be considered. Thecorrection circuit 30103 shown in FIG. 53D includes a LUT 30109 and anadder 30110. Difference data between the input image signal 30101 a andthe output image signal 30104 to be output is stored in the LUT 30109.That is, corresponding difference data between the input image signal30101 a and the input image signal 30101 b is extracted from the LUT30109, and the extracted difference data and the input image signal30101 a are added by the adder 30110; thus, the output image signal30104 can be obtained. Note that when data stored in the LUT 30109 isdifference data, memory capacity of the LUT can be reduced. This isbecause the size of difference data is smaller than that of the outputimage signal 30104 as it is, so that memory capacity necessary for theLUT 30109 can be reduced.

In addition, when the output image signal can be calculated by simpleoperation such as four arithmetic operations of the two input imagesignals, the correction circuit 30103 can be realized by a combinationof simple circuits such as an adder, a subtractor, and a multiplier.Accordingly, it is not necessary to use the LUT, and manufacturing costcan be significantly reduced. As such a circuit, a circuit shown in FIG.53E can be considered. The correction circuit 30103 shown in FIG. 53Eincludes a subtractor 30111, a multiplier 30112, and an adder 30113.First, difference between the input image signal 30101 a and the inputimage signal 30101 b is calculated by the subtractor 30111. After that,a differential value is multiplied by an appropriate coefficient byusing the multiplier 30112. Then, the differential value multiplied bythe appropriate coefficient is added to the input image signal 30101 aby the adder 30113; thus, the output image signal 30104 can be obtained.By using such a circuit, it is not necessary to use the LUT, so thatmanufacturing cost can be significantly reduced.

By using the correction circuit 30103 shown in FIG. 53E under a certaincondition, inappropriate output of the output image signal 30104 can beprevented. The condition is that the output image signal 30104 applyingthe overdriving voltage and a differential value between the input imagesignals 30101 a and 30101 b have linearity. The slope of this linearityis a coefficient to be multiplied by the multiplier 30112. That is, thecorrection circuit 30103 shown in FIG. 53E is preferably used for aliquid crystal element having such properties. An example of a liquidcrystal element having such properties includes an IPS mode liquidcrystal element in which response time has little gray-scale dependency.For example, when the correction circuit 30103 shown in FIG. 53E is usedfor an IPS mode liquid crystal element, manufacturing cost can besignificantly reduced, and an overdriving circuit which can preventoutput of the inappropriate output image signal 30104 can be obtained.

Note that operations which are similar to those of the circuit shown inFIGS. 53A to 53E may be realized by software processing. As the memoryused for the delay circuit, another memory included in the liquidcrystal display device, a memory included in a device which transfers animage displayed on the liquid crystal display device (e.g., a video cardor the like included in a personal computer or a device similar to thepersonal computer), or the like can be used. Accordingly, not only canmanufacturing cost be reduced, intensity of overdriving, availability,or the like can be selected in accordance with user's preference.

Next, driving which controls a potential of a common line is describedwith reference to FIGS. 54A and 54B. FIG. 54A is a diagram showing aplurality of pixel circuits in which one common line is provided withrespect to one scan line in a display device using a display elementwhich has capacitive properties like a liquid crystal element. Each ofthe pixel circuits shown in FIG. 54A includes a transistor 30201, anauxiliary capacitor 30202, a display element 30203, a video signal line30204, a scan line 30205, and a common line 30206.

A gate electrode of the transistor 30201 is electrically connected tothe scan line 30205; one of a source electrode and a drain electrode ofthe transistor 30201 is electrically connected to the video signal line30204; and the other of the source electrode and the drain electrode ofthe transistor 30201 is electrically connected to one of electrodes ofthe auxiliary capacitor 30202 and one of electrodes of the displayelement 30203. In addition, the other of the electrodes of the auxiliarycapacitor 30202 is electrically connected to the common line 30206.

First, in each of pixels selected by the scan line 30205, voltagecorresponding to an image signal is applied to the display element 30203and the auxiliary capacitor 30202 through the video signal line 30204because the transistor 30201 is turned on. At this time, when the imagesignal is a signal which makes all of pixels connected to the commonline 30206 display a minimum gray scale or when the image signal is asignal which makes all of the pixels connected to the common line 30206display a maximum gray scale, it is not necessary that the image signalbe written to each of the pixels through the video signal line 30204.Voltage applied to the display element 30203 can be changed by changinga potential of the common line 30206 instead of writing the image signalthrough the video signal line 30204.

Next, FIG. 54B is a diagram showing a plurality of pixel circuits inwhich two common lines are provided with respect to one scan line in adisplay device using a display element which has capacitive propertieslike a liquid crystal element. Each of the pixel circuits shown in FIG.54B includes a transistor 30211, an auxiliary capacitor 30212, a displayelement 30213, a video signal line 30214, a scan line 30215, a firstcommon line 30216, and a second common line 30217.

A gate electrode of the transistor 30211 is electrically connected tothe scan line 30215; one of a source electrode and a drain electrode ofthe transistor 30211 is electrically connected to the video signal line30214; and the other of the source electrode and the drain electrode ofthe transistor 30211 is electrically connected to one of electrodes ofthe auxiliary capacitor 30212 and one of electrodes of the displayelement 30213. In addition, the other of the electrodes of the auxiliarycapacitor 30212 is electrically connected to the first common line30216. Further, in a pixel which is adjacent to the pixel, the other ofthe electrodes of the auxiliary capacitor 30212 is electricallyconnected to the second common line 30217.

In the pixel circuits shown in FIG. 54B, the number of pixels which areelectrically connected to one common line is small. Therefore, bychanging a potential of the first common line 30216 or the second commonline 30217 instead of writing an image signal through the video signalline 30214, frequency of changing voltage applied to the display element30213 is significantly increased. In addition, source inversion drivingor dot inversion driving can be performed. By performing sourceinversion driving or dot inversion driving, reliability of the elementcan be improved and a flicker can be suppressed.

A scanning backlight is described with reference to FIGS. 55A to 55C.FIG. 55A is a view showing a scanning backlight in which cold cathodefluorescent lamps are arranged. The scanning backlight shown in FIG. 55Aincludes a diffusion plate 30301 and N pieces of cold cathodefluorescent lamps 30302-1 to 30302-N. The N pieces of the cold cathodefluorescent lamps 30302-1 to 30302-N are arranged on the back side ofthe diffusion plate 30301, so that the N pieces of the cold cathodefluorescent lamps 30302-1 to 30302-N can be scanned while luminancethereof is changed.

Change in luminance of each of the cold cathode fluorescent lamps inscanning is described with reference to FIG. 55C. First, luminance ofthe cold cathode fluorescent lamp 30302-1 is changed for a certainperiod. After that, luminance of the cold cathode fluorescent lamp30302-2 which is provided adjacent to the cold cathode fluorescent lamp30302-1 is changed for the same period. In this manner, luminance ischanged sequentially from the cold cathode fluorescent lamp 30302-1 tothe cold cathode fluorescent lamp 30302-N. Although luminance which ischanged for a certain period is set to be lower than original luminancein FIG. 55C, it may also be higher than original luminance. In addition,although scanning is performed from the cold cathode fluorescent lamps30302-1 to 30302-N, scanning may also be performed from the cold cathodefluorescent lamps 30302-N to 30302-1, which is in a reversed order.

By performing driving as in FIG. 55C, average luminance of the backlightcan be decreased. Therefore, power consumption of the backlight, whichmainly takes up power consumption of the liquid crystal display device,can be reduced.

Note that an LED may be used as a light source of the scanningbacklight. The scanning backlight in that case is as shown in FIG. 55B.The scanning backlight shown in FIG. 55B includes a diffusion plate30311 and light sources 30312-1 to 30312-N, in each of which LEDs arearranged. When the LED is used as the light source of the scanningbacklight, there is an advantage in that the backlight can be thin andlightweight. In addition, there is also an advantage that a colorreproduction area can be widened. Further, since the LEDs which arearranged in each of the light sources 30312-1 to 30312-N can besimilarly scanned, a dot scanning backlight can also be obtained. Byusing the dot scanning backlight, quality of a moving image can befurther improved.

Note that when the LED is used as the light source of the backlight,driving can be performed by changing luminance as shown in FIG. 55C.

Note that although this embodiment mode is described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, even more drawings can be formed by combining each part withanother part in the above-described drawings.

Similarly, the contents (or part of the contents) described in eachdrawing of this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, even more drawings can beformed by combining each part with part of another embodiment mode inthe drawings of this embodiment mode.

This embodiment mode shows an example of an embodied case of thecontents (or part of the contents) described in another embodiment mode,an example of slight transformation thereof, an example of partialmodification thereof, an example of improvement thereof, an example ofdetailed description thereof, an application example thereof, an exampleof related part thereof, or the like. Therefore, the contents describedin another embodiment mode can be freely applied to, combined with, orreplaced with this embodiment mode.

Embodiment Mode 12

In this embodiment mode, various liquid crystal modes are described.

First, various liquid crystal modes are described with reference tocross-sectional views.

FIGS. 58A and 58B are schematic views of cross sections of a TN mode.

A liquid crystal layer 50100 is held between a first substrate 50101 anda second substrate 50102 which are provided so as to be opposite to eachother. A first electrode 50105 is formed on a top surface of the firstsubstrate 50101. A second electrode 50106 is formed on a top surface ofthe second substrate 50102. A first polarizing plate 50103 is providedon a surface of the first substrate 50101, which does not face theliquid crystal layer. A second polarizing plate 50104 is provided on asurface of the second substrate 50102, which does not face the liquidcrystal layer. Note that the first polarizing plate 50103 and the secondpolarizing plate 50104 are provided so as to be in a cross nicol state.

The first polarizing plate 50103 may be provided on the top surface ofthe first substrate 50101. The second polarizing plate 50104 may beprovided on the top surface of the second substrate 50102.

It is acceptable as long as at least one of or both the first electrode50105 and the second electrode 50106 have light-transmitting properties(a transmissive or reflective liquid crystal display device).Alternatively, both the first electrode 50105 and the second electrode50106 may have light-transmitting properties, and part of one of theelectrodes may have reflectivity (a semi-transmissive liquid crystaldisplay device).

FIG. 58A is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 50105 and the second electrode50106 (referred to as a vertical electric field mode). Since liquidcrystal molecules are aligned longitudinally, light emitted from abacklight is not affected by birefringence of the liquid crystalmolecules. In addition, since the first polarizing plate 50103 and thesecond polarizing plate 50104 are provided so as to be in a cross nicolstate, light emitted from the backlight cannot pass through thesubstrate. Therefore, black display is performed.

Note that by controlling voltage applied to the first electrode 50105and the second electrode 50106, conditions of the liquid crystalmolecules can be controlled. Therefore, since the amount of lightemitted from the backlight passing through the substrate can becontrolled, predetermined image display can be performed.

FIG. 58B is a schematic view of a cross section in the case wherevoltage is not applied to the first electrode 50105 and the secondelectrode 50106. Since the liquid crystal molecules are alignedlaterally and rotated in a plane, light emitted from a backlight isaffected by birefringence of the liquid crystal molecules. In addition,since the first polarizing plate 50103 and the second polarizing plate50104 are provided so as to be in a cross nicol state, light emittedfrom the backlight passes through the substrate. Therefore, whitedisplay is performed. This is a so-called normally white mode.

A liquid crystal display device having a structure shown in FIG. 58A orFIG. 58B can perform full-color display by being provided with a colorfilter. The color filter can be provided on a first substrate 50101 sideor a second substrate 50102 side.

It is acceptable as long as a known material is used for a liquidcrystal material used for a TN mode.

FIGS. 59A and 59B are schematic views of cross sections of a VA mode. Inthe VA mode, liquid crystal molecules are aligned such that they arevertical to a substrate when there is no electric field.

A liquid crystal layer 50200 is held between a first substrate 50201 anda second substrate 50202 which are provided so as to be opposite to eachother. A first electrode 50205 is formed on a top surface of the firstsubstrate 50201. A second electrode 50206 is formed on a top surface ofthe second substrate 50202. A first polarizing plate 50203 is providedon a surface of the first substrate 50201, which does not face theliquid crystal layer. A second polarizing plate 50204 is provided on asurface of the second substrate 50202, which does not face the liquidcrystal layer. Note that the first polarizing plate 50203 and the secondpolarizing plate 50204 are provided so as to be in a cross nicol state.

The first polarizing plate 50203 may be provided on the top surface ofthe first substrate 50201. The second polarizing plate 50204 may beprovided on the top surface of the second substrate 50202.

It is acceptable as long as at least one of or both the first electrode50205 and the second electrode 50206 have light-transmitting properties(a transmissive or reflective liquid crystal display device).Alternatively, both the first electrode 50205 and the second electrode50206 may have light-transmitting properties, and part of one of theelectrodes may have reflectivity (a semi-transmissive liquid crystaldisplay device).

FIG. 59A is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 50205 and the second electrode50206 (referred to as a vertical electric field mode). Since liquidcrystal molecules are aligned laterally, light emitted from a backlightis affected by birefringence of the liquid crystal molecules. Inaddition, since the first polarizing plate 50203 and the secondpolarizing plate 50204 are provided so as to be in a cross nicol state,light emitted from the backlight passes through the substrate.Therefore, white display is performed.

Note that by controlling voltage applied to the first electrode 50205and the second electrode 50206, conditions of the liquid crystalmolecules can be controlled. Therefore, since the amount of lightemitted from the backlight passing through the substrate can becontrolled, predetermined image display can be performed.

FIG. 59B is a schematic view of a cross section in the case wherevoltage is not applied to the first electrode 50205 and the secondelectrode 50206. Since liquid crystal molecules are alignedlongitudinally, light emitted from a backlight is not affected bybirefringence of the liquid crystal molecules. In addition, since thefirst polarizing plate 50203 and the second polarizing plate 50204 areprovided so as to be in a cross nicol state, light emitted from thebacklight does not pass through the substrate. Therefore, black displayis performed. This is a so-called normally black mode.

A liquid crystal display device having a structure shown in FIG. 59A orFIG. 59B can perform full-color display by being provided with a colorfilter. The color filter can be provided on a first substrate 50201 sideor a second substrate 50202 side.

It is acceptable as long as a known material is used for a liquidcrystal material used for a VA mode.

FIGS. 59C and 59D are schematic views of cross sections of an MVA mode.In the MVA mode, viewing angle dependency of each portion is compensatedby each other.

A liquid crystal layer 50210 is held between a first substrate 50211 anda second substrate 50212 which are provided so as to be opposite to eachother. A first electrode 50215 is formed on a top surface of the firstsubstrate 50211. A second electrode 50216 is formed on a top surface ofthe second substrate 50212. A first protrusion 50217 for controllingalignment is formed on the first electrode 50215. A second protrusion50218 for controlling alignment is formed over the second electrode50216. A first polarizing plate 50213 is provided on a surface of thefirst substrate 50211, which does not face the liquid crystal layer. Asecond polarizing plate 50214 is provided on a surface of the secondsubstrate 50212, which does not face the liquid crystal layer. Note thatthe first polarizing plate 50213 and the second polarizing plate 50214are provided so as to be in a cross nicol state.

The first polarizing plate 50213 may be provided on the top surface ofthe first substrate 50211. The second polarizing plate 50214 may beprovided on the top surface of the second substrate 50212.

It is acceptable as long as at least one of or both the first electrode50215 and the second electrode 50216 have light-transmitting properties(a transmissive or reflective liquid crystal display device).Alternatively, both the first electrode 50215 and the second electrode50216 may have light-transmitting properties, and part of one of theelectrodes may have reflectivity (a semi-transmissive liquid crystaldisplay device).

FIG. 59C is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 50215 and the second electrode50216 (referred to as a vertical electric field mode). Since liquidcrystal molecules are aligned so as to tilt toward the first protrusion50217 and the second protrusion 50218, light emitted from a backlight isaffected by birefringence of the liquid crystal molecules. In addition,since the first polarizing plate 50213 and the second polarizing plate50214 are provided so as to be in a cross nicol state, light emittedfrom the backlight passes through the substrate. Therefore, whitedisplay is performed.

Note that by controlling voltage applied to the first electrode 50215and the second electrode 50216, conditions of the liquid crystalmolecules can be controlled. Therefore, since the amount of lightemitted from the backlight passing through the substrate can becontrolled, predetermined image display can be performed.

FIG. 59D is a schematic view of a cross section in the case wherevoltage is not applied to the first electrode 50215 and the secondelectrode 50216. Since liquid crystal molecules are alignedlongitudinally, light emitted from a backlight is not affected bybirefringence of the liquid crystal molecules. In addition, since thefirst polarizing plate 50213 and the second polarizing plate 50214 areprovided so as to be in a cross nicol state, light emitted from thebacklight does not pass through the substrate. Therefore, black displayis performed. This is a so-called normally black mode.

A liquid crystal display device having a structure shown in FIG. 59C orFIG. 59D can perform full-color display by being provided with a colorfilter. The color filter can be provided on a first substrate 50211 sideor a second substrate 50212 side. It is acceptable as long as a knownmaterial is used for a liquid crystal material used for an MVA mode.

FIGS. 60A and 60B are schematic views of cross sections of an OCB mode.In the OCB mode, viewing angle dependency is low because alignment ofliquid crystal molecules in a liquid crystal layer can be opticallycompensated. This state of the liquid crystal molecules is referred toas bend alignment.

A liquid crystal layer 50300 is held between a first substrate 50301 anda second substrate 50302 which are provided so as to be opposite to eachother. A first electrode 50305 is formed on a top surface of the firstsubstrate 50301. A second electrode 50306 is formed on a top surface ofthe second substrate 50302. A first polarizing plate 50303 is providedon a surface of the first substrate 50301, which does not face theliquid crystal layer. A second polarizing plate 50304 is provided on asurface of the second substrate 50302, which does not face the liquidcrystal layer. Note that the first polarizing plate 50303 and the secondpolarizing plate 50304 are provided so as to be in a cross nicol state.

The first polarizing plate 50303 may be provided on the top surface ofthe first substrate 50301. The second polarizing plate 50304 may beprovided on the top surface of the second substrate 50302.

It is acceptable as long as at least one of or both the first electrode50305 and the second electrode 50306 have light-transmitting properties(a transmissive or reflective liquid crystal display device).Alternatively, both the first electrode 50305 and the second electrode50306 may have light-transmitting properties, and part of one of theelectrodes may have reflectivity (a semi-transmissive liquid crystaldisplay device).

FIG. 60A is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 50305 and the second electrode50306 (referred to as a vertical electric field mode). Since liquidcrystal molecules are aligned longitudinally, light emitted from abacklight is not affected by birefringence of the liquid crystalmolecules. In addition, since the first polarizing plate 50303 and thesecond polarizing plate 50304 are provided so as to be in a cross nicolstate, light emitted from the backlight does not pass through thesubstrate. Therefore, black display is performed.

Note that by controlling voltage applied to the first electrode 50305and the second electrode 50306, conditions of the liquid crystalmolecules can be controlled. Therefore, since the amount of lightemitted from the backlight passing through the substrate can becontrolled, predetermined image display can be performed.

FIG. 60B is a schematic view of a cross section in the case wherevoltage is not applied to the first electrode 50305 and the secondelectrode 50306. Since liquid crystal molecules are in a bend alignmentstate, light emitted from a backlight is affected by birefringence ofthe liquid crystal molecules. In addition, since the first polarizingplate 50303 and the second polarizing plate 50304 are provided so as tobe in a cross nicol state, light emitted from the backlight passesthrough the substrate. Therefore, white display is performed. This is aso-called normally white mode.

A liquid crystal display device having a structure shown in FIG. 60A orFIG. 60B can perform full-color display by being provided with a colorfilter. The color filter can be provided on a first substrate 50301 sideor a second substrate 50302 side.

It is acceptable as long as a known material is used for a liquidcrystal material used for an OCB mode.

FIGS. 60C and 60D are schematic views of cross sections of an FLC modeor an AFLC mode.

A liquid crystal layer 50310 is held between a first substrate 50311 anda second substrate 50312 which are provided so as to be opposite to eachother. A first electrode 50315 is formed on a top surface of the firstsubstrate 50311. A second electrode 50316 is formed on a top surface ofthe second substrate 50312. A first polarizing plate 50313 is providedon a surface of the first substrate 50311, which does not face theliquid crystal layer. A second polarizing plate 50314 is provided on asurface of the second substrate 50312, which does not face the liquidcrystal layer. Note that the first polarizing plate 50313 and the secondpolarizing plate 50314 are provided so as to be in a cross nicol state.

The first polarizing plate 50313 may be provided on the top surface ofthe first substrate 50311. The second polarizing plate 50314 may beprovided on the top surface of the second substrate 50312.

It is acceptable as long as at least one of or both the first electrode50315 and the second electrode 50316 have light-transmitting properties(a transmissive or reflective liquid crystal display device).Alternatively, both the first electrode 50315 and the second electrode50316 may have light-transmitting properties, and part of one of theelectrodes may have reflectivity (a semi-transmissive liquid crystaldisplay device).

FIG. 60C is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 50315 and the second electrode50316 (referred to as a vertical electric field mode). Since liquidcrystal molecules are aligned laterally in a direction which is deviatedfrom a rubbing direction, light emitted from a backlight is affected bybirefringence of the liquid crystal molecules. In addition, since thefirst polarizing plate 50313 and the second polarizing plate 50314 areprovided so as to be in a cross nicol state, light emitted from thebacklight passes through the substrate. Therefore, white display isperformed.

Note that by controlling voltage applied to the first electrode 50315and the second electrode 50316, conditions of the liquid crystalmolecules can be controlled. Therefore, since the amount of lightemitted from the backlight passing through the substrate can becontrolled, predetermined image display can be performed.

FIG. 60D is a schematic view of a cross section in the case wherevoltage is not applied to the first electrode 50315 and the secondelectrode 50316. Since liquid crystal molecules are aligned laterally ina rubbing direction, light emitted from a backlight is not affected bybirefringence of the liquid crystal molecules. In addition, since thefirst polarizing plate 50313 and the second polarizing plate 50314 areprovided so as to be in a cross nicol state, light emitted from thebacklight does not pass through the substrate. Therefore, black displayis performed. This is a so-called normally black mode.

A liquid crystal display device having a structure shown in FIG. 60C orFIG. 60D can perform full-color display by being provided with a colorfilter. The color filter can be provided on a first substrate 50311 sideor a second substrate 50312 side.

It is acceptable as long as a known material is used for a liquidcrystal material used for an FLC mode or an AFLC mode.

FIGS. 61A and 61B are schematic views of cross sections of an IPS mode.In the IPS mode, alignment of liquid crystal molecules in a liquidcrystal layer can be optically compensated, the liquid crystal moleculesare constantly rotated in a plane parallel to a substrate, and ahorizontal electric field method in which electrodes are provided onlyon one substrate side is used.

A liquid crystal layer 50400 is held between a first substrate 50401 anda second substrate 50402 which are provided so as to be opposite to eachother. A first electrode 50405 and a second electrode 50406 are formedon a top surface of the second substrate 50402. A first polarizing plate50403 is provided on a surface of the first substrate 50401, which doesnot face the liquid crystal layer. A second polarizing plate 50404 isprovided on a surface of the second substrate 50402, which does not facethe liquid crystal layer. Note that the first polarizing plate 50403 andthe second polarizing plate 50404 are provided so as to be in a crossnicol state.

The first polarizing plate 50403 may be provided on the top surface ofthe first substrate 50401. The second polarizing plate 50404 may beprovided on the top surface of the second substrate 50402.

It is acceptable as long as both the first electrode 50405 and thesecond electrode 50406 have light-transmitting properties.Alternatively, part of one of the first electrode 50405 and the secondelectrode 50406 may have reflectivity.

FIG. 61A is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 50405 and the second electrode50406 (referred to as a horizontal electric field mode). Since liquidcrystal molecules are aligned along a line of electric force which isdeviated from a rubbing direction, light emitted from a backlight isaffected by birefringence of the liquid crystal molecules. In addition,since the first polarizing plate 50403 and the second polarizing plate50404 are provided so as to be in a cross nicol state, light emittedfrom the backlight passes through the substrate. Therefore, whitedisplay is performed.

Note that by controlling voltage applied to the first electrode 50405and the second electrode 50406, conditions of the liquid crystalmolecules can be controlled. Therefore, since the amount of lightemitted from the backlight passing through the substrate can becontrolled, predetermined image display can be performed.

FIG. 61B is a schematic view of a cross section in the case wherevoltage is not applied to the first electrode 50405 and the secondelectrode 50406. Since liquid crystal molecules are aligned laterally ina rubbing direction, light emitted from a backlight is not affected bybirefringence of the liquid crystal molecules. In addition, since thefirst polarizing plate 50403 and the second polarizing plate 50404 areprovided so as to be in a cross nicol state, light emitted from thebacklight does not pass through the substrate. Therefore, black displayis performed. This is a so-called normally black mode.

A liquid crystal display device having a structure shown in FIG. 61A orFIG. 61B can perform full-color display by being provided with a colorfilter. The color filter can be provided on a first substrate 50401 sideor a second substrate 50402 side.

It is acceptable as long as a known material is used for a liquidcrystal material used for an IPS mode.

FIGS. 61C and 61D are schematic views of cross sections of an FFS mode.In the FFS mode, alignment of liquid crystal molecules in a liquidcrystal layer can be optically compensated, the liquid crystal moleculesare constantly rotated in a plane parallel to a substrate, and ahorizontal electric field method in which electrodes are provided onlyon one substrate side is used.

A liquid crystal layer 50410 is held between a first substrate 50411 anda second substrate 50412 which are provided so as to be opposite to eachother. A second electrode 50416 is formed on a top surface of the secondsubstrate 50412. An insulating film 50417 is formed on a top surface ofthe second electrode 50416. A first electrode 50415 is formed over theinsulating film 50417. A first polarizing plate 50413 is provided on asurface of the first substrate 50411, which does not face the liquidcrystal layer. A second polarizing plate 50414 is provided on a surfaceof the second substrate 50412, which does not face the liquid crystallayer. Note that the first polarizing plate 50413 and the secondpolarizing plate 50414 are provided so as to be in a cross nicol state.

The first polarizing plate 50413 may be provided on the top surface ofthe first substrate 50411. The second polarizing plate 50414 may beprovided on the top surface of the second substrate 50412.

It is acceptable as long as at least one of or both the first electrode50415 and the second electrode 50416 have light-transmitting properties(a transmissive or reflective liquid crystal display device).Alternatively, both the first electrode 50415 and the second electrode50416 may have light-transmitting properties, and part of one of theelectrodes may have reflectivity (a semi-transmissive liquid crystaldisplay device).

FIG. 61C is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 50415 and the second electrode50416 (referred to as a horizontal electric field mode). Since liquidcrystal molecules are aligned along a line of electric force which isdeviated from a rubbing direction, light emitted from a backlight isaffected by birefringence of the liquid crystal molecules. In addition,since the first polarizing plate 50413 and the second polarizing plate50414 are provided so as to be in a cross nicol state, light emittedfrom the backlight passes through the substrate. Therefore, whitedisplay is performed.

Note that by controlling voltage applied to the first electrode 50415and the second electrode 50416, conditions of the liquid crystalmolecules can be controlled. Therefore, since the amount of lightemitted from the backlight passing through the substrate can becontrolled, predetermined image display can be performed.

FIG. 61D is a schematic view of a cross section in the case wherevoltage is not applied to the first electrode 50415 and the secondelectrode 50416. Since liquid crystal molecules are aligned laterally ina rubbing direction, light emitted from the backlight is not affected bybirefringence of the liquid crystal molecules. In addition, since thefirst polarizing plate 50413 and the second polarizing plate 50414 areprovided so as to be in a cross nicol state, light emitted from thebacklight does not pass through the substrate. Therefore, black displayis performed. This is a so-called normally black mode.

A liquid crystal display device having a structure shown in FIG. 61C orFIG. 61D can perform full-color display by being provided with a colorfilter. The color filter can be provided on a first substrate 50411 sideor a second substrate 50412 side.

It is acceptable as long as a known material is used for a liquidcrystal material used for an FFS mode.

Next, various liquid crystal modes are described with reference to topplan views.

FIG. 62 is a top plan view of a pixel portion to which an MVA mode isapplied. In the MVA mode, viewing angle dependency of each portion iscompensated by each other.

FIG. 62 shows a first pixel electrode 50501, second pixel electrodes(50502 a, 50502 b, and 50502 c), and a protrusion 50503. The first pixelelectrode 50501 is formed over the entire surface of a countersubstrate. The protrusion 50503 is formed so as to be a dogleg shape. Inaddition, the second pixel electrodes (50502 a, 50502 b, and 50502 c)are formed over the first pixel electrode 50501 so as to have shapescorresponding to the protrusion 50503.

Opening portions of the second pixel electrodes (50502 a, 50502 b, and50502 c) function like protrusions.

In the case where voltage is applied to the first pixel electrode 50501and the second pixel electrodes (50502 a, 50502 b, and 50502 c)(referred to as a vertical electric field mode), liquid crystalmolecules are aligned so as to tilt toward the opening portions of thesecond pixel electrodes (50502 a, 50502 b, and 50502 c) and theprotrusion 50503. Since light emitted from a backlight passes through asubstrate when a pair of polarizing plates is provided so as to be in across nicol state, white display is performed.

Note that by controlling voltage applied to the first pixel electrode50501 and the second pixel electrodes (50502 a, 50502 b, and 50502 c),conditions of the liquid crystal molecules can be controlled. Therefore,since the amount of light emitted from the backlight passing through thesubstrate can be controlled, predetermined image display can beperformed.

In the case where voltage is not applied to the first pixel electrode50501 and the second pixel electrodes (50502 a, 50502 b, and 50502 c),the liquid crystal molecules are aligned longitudinally. Since lightemitted from the backlight does not pass through a panel when the pairof polarizing plates is provided so as to be in the cross nicol state,black display is performed. This is a so-called normally black mode.

It is acceptable as long as a known material is used for a liquidcrystal material used for an MVA mode.

FIGS. 44A to 44D are top plan views of a pixel portion to which an IPSmode is applied. In the IPS mode, alignment of liquid crystal moleculesin a liquid crystal layer can be optically compensated, the liquidcrystal molecules are constantly rotated in a plane parallel to asubstrate, and a horizontal electric field method in which electrodesare provided only on one substrate side is used.

In the IPS mode, a pair of electrodes is formed so as to have differentshapes.

FIG. 63A shows a first pixel electrode 50601 and a second pixelelectrode 50602. The first pixel electrode 50601 and the second pixelelectrode 50602 are wavy shapes.

FIG. 63B shows a first pixel electrode 50611 and a second pixelelectrode 50612. The first pixel electrode 50611 and the second pixelelectrode 50612 have shapes having concentric openings.

FIG. 63C shows a first pixel electrode 50621 and a second pixelelectrode 50622. The first pixel electrode 50621 and the second pixelelectrode 50622 are comb shapes and partially overlap with each other.

FIG. 63D shows a first pixel electrode 50631 and a second pixelelectrode 50632. The first pixel electrode 50631 and the second pixelelectrode 50632 are comb shapes in which electrodes engage with eachother.

In the case where voltage is applied to the first pixel electrodes(50601, 50611, 50621, and 50631) and the second pixel electrodes (50602,50612, 50622, and 50623) (referred to as a horizontal electric fieldmode), liquid crystal molecules are aligned along a line of electricforce which is deviated from a rubbing direction. Since light emittedfrom a backlight passes through a substrate when a pair of polarizingplates is provided so as to be in a cross nicol state, white display isperformed.

Note that by controlling voltage applied to the first pixel electrodes(50601, 50611, 50621, and 50631) and the second pixel electrodes (50602,50612, 50622, and 50623), conditions of the liquid crystal molecules canbe controlled. Therefore, since the amount of light emitted from thebacklight passing through the substrate can be controlled, predeterminedimage display can be performed.

In the case where voltage is not applied to the first pixel electrodes(50601, 50611, 50621, and 50631) and the second pixel electrodes (50602,50612, 50622, and 50623), the liquid crystal molecules are alignedlaterally in the rubbing direction. Since light emitted from thebacklight does not pass through the substrate when the pair ofpolarizing plates is provided so as to be in the cross nicol state,black display is performed. This is a so-called normally black mode.

It is acceptable as long as a known material be used for a liquidcrystal material used for an IPS mode.

FIGS. 64A to 64D are top plan views of a pixel portion to which an FFSmode is applied. In the FFS mode, alignment of liquid crystal moleculesin a liquid crystal layer can be optically compensated, the liquidcrystal molecules are constantly rotated in a plane parallel to asubstrate, and a horizontal electric field method in which electrodesare provided only on one substrate side is used.

In the FFS mode, a first electrode is formed over a top surface of asecond electrode so as to be various shapes.

FIG. 64A shows a first pixel electrode 50701 and a second pixelelectrode 50702. The first pixel electrode 50701 is a bent dogleg shape.The second pixel electrode 50702 is not necessarily patterned.

FIG. 64B shows a first pixel electrode 50711 and a second pixelelectrode 50712. The first pixel electrode 50711 is a concentric shape.The second pixel electrode 50712 is not necessarily patterned.

FIG. 64C shows a first pixel electrode 50721 and a second pixelelectrode 50722. The first pixel electrode 50721 is a winding shape. Thesecond pixel electrode 50722 is not necessarily patterned.

FIG. 64D shows a first pixel electrode 50731 and a second pixelelectrode 50732. The first pixel electrode 50731 is a comb shape. Thesecond pixel electrode 50732 is not necessarily patterned.

In the case where voltage is applied to the first pixel electrodes(50701, 50711, 50721, and 50731) and the second pixel electrodes (50702,50712, 50722, and 50732) (referred to as a horizontal electric fieldmode), liquid crystal molecules are aligned along a line of electricforce which is deviated from a rubbing direction. Since light emittedfrom a backlight passes through a substrate when a pair of polarizingplates is provided so as to be in a cross nicol state, white display isperformed.

Note that by controlling voltage applied to the first pixel electrodes(50701, 50711, 50721, and 50731) and the second pixel electrodes (50702,50712, 50722, and 50732), conditions of the liquid crystal molecules canbe controlled. Therefore, since the amount of light emitted from thebacklight passing through the substrate can be controlled, predeterminedimage display can be performed.

In the case where voltage is not applied to the first pixel electrodes(50701, 50711, 50721, and 50731) and the second pixel electrodes (50702,50712, 50722, and 50732), the liquid crystal molecules are alignedlaterally in the rubbing direction. Since light emitted from thebacklight does not pass through the substrate when the pair ofpolarizing plates is provided so as to be in the cross nicol state,black display is performed. This is a so-called normally black mode.

It is acceptable as long as a known material is used for a liquidcrystal material used for an FFS mode.

Note that although this embodiment mode is described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, even more drawings can be formed by combining each part withanother part in the above-described drawings.

Similarly, the contents (or part of the contents) described in eachdrawing of this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, even more drawings can beformed by combining each part with part of another embodiment mode inthe drawings of this embodiment mode.

This embodiment mode shows an example of an embodied case of thecontents (or part of the contents) described in another embodiment mode,an example of slight transformation thereof, an example of partialmodification thereof, an example of improvement thereof, an example ofdetailed description thereof, an application example thereof, an exampleof related part thereof, or the like. Therefore, the contents describedin another embodiment mode can be freely applied to, combined with, orreplaced with this embodiment mode.

Embodiment Mode 13

In this embodiment mode, a pixel structure of a display device isdescribed. In particular, a pixel structure of a liquid crystal displaydevice is described.

A pixel structure in the case where each liquid crystal mode and atransistor are combined is described with reference to cross-sectionalviews of a pixel.

Note that as the transistor, a thin film transistor (a TFT) or the likeincluding a non-single crystalline semiconductor layer typified by anamorphous silicon layer, a polycrystalline silicon layer, amicrocrystalline (also referred to as semi-amorphous) silicon layer, orthe like can be used.

As a structure of the transistor, a top-gate structure, a bottom-gatestructure, or the like can be used. Note that a channel-etchedtransistor, a channel-protective transistor, or the like can be used asa bottom-gate transistor.

FIG. 65 is an example of a cross-sectional view of a pixel in the casewhere a TN mode and a transistor are combined. By applying the pixelstructure shown in FIG. 65 to a liquid crystal display device, a liquidcrystal display device can be formed at low cost.

Features of the pixel structure shown in FIG. 65 are described. Liquidcrystal molecules 10118 shown in FIG. 65 are long and narrow moleculeseach having a major axis and a minor axis. In FIG. 65, a direction ofeach of the liquid crystal molecules 10118 is expressed by the lengththereof. That is, the direction of the major axis of the liquid crystalmolecule 10118, which is expressed as long, is parallel to the page, andas the liquid crystal molecule 10118 is expressed to be shorter, thedirection of the major axis becomes closer to a normal direction of thepage. That is, among the liquid crystal molecules 10118 shown in FIG.65, the direction of the major axis of the liquid crystal molecule 10118which is close to a first substrate 10101 and the direction of the majoraxis of the liquid crystal molecule 10118 which is close to a secondsubstrate 10116 are different from each other by 90 degrees, and thedirections of the major axes of the liquid crystal molecules 10118located therebetween are arranged so as to link the above two directionssmoothly. That is, the liquid crystal molecules 10118 shown in FIG. 65are aligned to be twisted by 90 degrees between the first substrate10101 and the second substrate 10116.

Note that the case is described in which a bottom-gate transistor usingan amorphous semiconductor is used as the transistor. In the case wherea transistor using an amorphous semiconductor is used, a liquid crystaldisplay device can be formed at low cost by using a large substrate.

A liquid crystal display device includes a basic portion for displayingimages, which is called a liquid crystal panel. The liquid crystal panelis manufactured as follows: two processed substrates are attached toeach other with a gap of several μm therebetween, and a liquid crystalmaterial is injected into a space between the two substrates. In FIG.65, the two substrates correspond to the first substrate 10101 and thesecond substrate 10116. A transistor and a pixel electrode are formedover the first substrate. A light-shielding film 10114, a color filter10115, a fourth conductive layer 10113, a spacer 10117, and a secondalignment film 10112 are formed over the second substrate.

The light-shielding film 10114 is not necessarily formed over the secondsubstrate 10116. When the light-shielding film 10114 is not formed, thenumber of steps is reduced, so that manufacturing cost can be reduced.Further, since a structure is simple, yield can be improved.Alternatively, when the light-shielding film 10114 is formed, a displaydevice with less light leakage at the time of black display can beobtained.

The color filter 10115 is not necessarily formed over the secondsubstrate 10116. When the color filter 10115 is not formed, the numberof steps is reduced, so that manufacturing cost can be reduced. Further,since a structure is simple, yield can be improved. Note that even whenthe color filter 10115 is not formed, a display device which can performcolor display can be obtained by field sequential driving.Alternatively, when the color filter 10115 is formed, a display devicewhich can perform color display can be obtained.

Spherical spacers may be dispersed instead of forming the spacer 10117.When the spherical spacers are dispersed, the number of steps isreduced, so that manufacturing cost can be reduced. In addition, since astructure is simple when the spherical spacers are dispersed, yield canbe improved. Alternatively, when the spacer 10117 is formed, a distancebetween the two substrates can be uniform because a position of thespacer is not varied, so that a display device with little displayunevenness can be obtained.

A process to be performed to the first substrate 10101 is describedbelow.

First, a first insulating film 10102 is formed over the first substrate10101 by sputtering, a printing method, a coating method, or the like.Note that the first insulating film 10102 is not necessarily formed. Thefirst insulating film 10102 has a function of preventing change incharacteristics of the transistor due to an adverse effect of animpurity from the first substrate 10101 on a semiconductor layer.

Next, a first conductive layer 10103 is formed over the first insulatingfilm 10102 by photolithography, a laser direct writing method, an inkjetmethod, or the like.

Next, a second insulating film 10104 is formed over the entire surfaceby sputtering, a printing method, a coating method, or the like. Thesecond insulating film 10104 has a function of preventing change incharacteristics of the transistor due to an adverse effect of animpurity from the first substrate 10101 on the semiconductor layer.

Next, a first semiconductor layer 10105 and a second semiconductor layer10106 are formed. Note that the first semiconductor layer 10105 and thesecond semiconductor layer 10106 are formed sequentially and shapesthereof are processed at the same time.

Next, a second conductive layer 10107 is formed by photolithography, alaser direct writing method, an inkjet method, or the like. Note that asa method for etching which is performed at the time of processing ashape of the second conductive layer 10107, dry etching is preferable.Note that either a light-transmitting material or a reflective materialmay be used for the second conductive layer 10107.

Next, a channel formation region of the transistor is formed. Here, anexample of a step thereof is described. The second semiconductor layer10106 is etched by using the second conductive layer 10107 as a mask.Alternatively, the second semiconductor layer 10106 is etched by using amask for processing the shape of the second conductive layer 10107.Then, the first conductive layer 10103 at a position where the secondsemiconductor layer 10106 is removed serves as the channel formationregion of the transistor. Thus, the number of masks can be reduced, sothat manufacturing cost can be reduced.

Next, a third insulating film 10108 is formed and a contact hole isselectively formed in the third insulating film 10108. Note that acontact hole may be formed also in the second insulating film 10104 atthe same time as forming the contact hole in the third insulating film10108. Note that a surface of the third insulating film 10108 ispreferably as even as possible. This is because alignment of the liquidcrystal molecules are affected by roughness of a surface with which theliquid crystal is in contact.

Next, a third conductive layer 10109 is formed by photolithography, alaser direct writing method, an inkjet method, or the like.

Next, a first alignment film 10110 is formed. Note that after the firstalignment film 10110 is formed, rubbing may be performed so as tocontrol the alignment of the liquid crystal molecules. Rubbing is a stepof forming stripes on an alignment film by rubbing the alignment filmwith a cloth. By performing rubbing, the alignment film can havealignment properties.

The first substrate 10101 which is manufactured as described above andthe second substrate 10116 provided with the light-shielding film 10114,the color filter 10115, the fourth conductive layer 10113, the spacer10117, and the second alignment film 10112 are attached to each other bya sealant with a gap of several μm therebetween. Then, a liquid crystalmaterial is injected into a space between the two substrates. Note thatin the TN mode, the fourth conductive layer 10113 is formed over theentire surface of the second substrate 10116.

FIG. 66A is an example of a cross-sectional view of a pixel in the casewhere an MVA (multi-domain vertical alignment) mode and a transistor arecombined. By applying the pixel structure shown in FIG. 66A to a liquidcrystal display device, a liquid crystal display device having a wideviewing angle, high response speed, and high contrast can be obtained.

Features of the pixel structure shown in FIG. 66A are described below.Features of the pixel structure of an MVA mode liquid crystal panel aredescribed. Liquid crystal molecules 10218 shown in FIG. 66A are long andnarrow molecules each having a major axis and a minor axis. In FIG. 66A,a direction of each of the liquid crystal molecules 10218 is expressedby the length thereof. That is, the direction of the major axis of theliquid crystal molecule 10218, which is expressed as long, is parallelto the page, and as the liquid crystal molecule 10218 is expressed to beshorter, the direction of the major axis becomes closer to a normaldirection of the page. That is, each of the liquid crystal molecules10218 shown in FIG. 66A is aligned such that the direction of the majoraxis is normal to the alignment film. Thus, the liquid crystal molecules10218 at a position where an alignment control protrusion 10219 isformed are aligned radially with the alignment control protrusion 10219as a center. With this state, a liquid crystal display device having awide viewing angle can be obtained.

Note that the case is described in which a bottom-gate transistor usingan amorphous semiconductor is used as the transistor. In the case wherea transistor using an amorphous semiconductor is used, a liquid crystaldisplay device can be formed at low cost by using a large substrate.

A liquid crystal display device includes a basic portion for displayingimages, which is called a liquid crystal panel. The liquid crystal panelis manufactured as follows: two processed substrates are attached toeach other with a gap of several μm therebetween, and a liquid crystalmaterial is injected into a space between the two substrates. In FIG.66A, the two substrates correspond to a first substrate 10201 and asecond substrate 10216. A transistor and a pixel electrode are formedover the first substrate. A light-shielding film 10214, a color filter10215, a fourth conductive layer 10213, a spacer 10217, a secondalignment film 10212, and the alignment control protrusion 10219 areformed over the second substrate.

The light-shielding film 10214 is not necessarily formed over the secondsubstrate 10216. When the light-shielding film 10214 is not formed, thenumber of steps is reduced, so that manufacturing cost can be reduced.Further, since a structure is simple, yield can be improved.Alternatively, when the light-shielding film 10214 is formed, a displaydevice with less light leakage at the time of black display can beobtained.

The color filter 10215 is not necessarily formed over the secondsubstrate 10216. When the color filter 10215 is not formed, the numberof steps is reduced, so that manufacturing cost can be reduced. Further,since a structure is simple, yield can be improved. Note that even whenthe color filter 10215 is not formed, a display device which can performcolor display can be obtained by field sequential driving.Alternatively, when the color filter 10215 is formed, a display devicewhich can perform color display can be obtained.

Spherical spacers may be dispersed instead of forming the spacer 10217.When the spherical spacers are dispersed, the number of steps isreduced, so that manufacturing cost can be reduced. In addition, since astructure is simple when the spherical spacers are dispersed, yield canbe improved. Alternatively, when the spacer 10217 is formed, a distancebetween the two substrates can be uniform because a position of thespacer is not varied, so that a display device with little displayunevenness can be obtained.

A process to be performed to the first substrate 10201 is describedbelow.

First, a first insulating film 10202 is formed over the first substrate10201 by sputtering, a printing method, a coating method, or the like.Note that the first insulating film 10202 is not necessarily formed. Thefirst insulating film 10202 has a function of preventing change incharacteristics of the transistor due to an adverse effect of animpurity from the first substrate 10201 on a semiconductor layer.

Next, a first conductive layer 10203 is formed over the first insulatingfilm 10202 by photolithography, a laser direct writing method, an inkjetmethod, or the like.

Next, a second insulating film 10204 is formed over the entire surfaceby sputtering, a printing method, a coating method, or the like. Thesecond insulating film 10204 has a function of preventing change incharacteristics of the transistor due to an adverse effect of animpurity from the first substrate 10201 on the semiconductor layer.

Next, a first semiconductor layer 10205 and a second semiconductor layer10206 are formed. Note that the first semiconductor layer 10205 and thesecond semiconductor layer 10206 are formed sequentially and shapesthereof are processed at the same time.

Next, a second conductive layer 10207 is formed by photolithography, alaser direct writing method, an inkjet method, or the like. Note that asa method for etching which is performed at the time of processing ashape of the second conductive layer 10207, dry etching is preferable.Note that as the second conductive layer 10207, either alight-transmitting material or a reflective material may be used.

Next, a channel formation region of the transistor is formed. Here, anexample of a step thereof is described. The second semiconductor layer10206 is etched by using the second conductive layer 10207 as a mask.Alternatively, the second semiconductor layer 10206 is etched by using amask for processing the shape of the second conductive layer 10207.Then, the first conductive layer 10203 at a position where the secondsemiconductor layer 10206 is removed serves as the channel formationregion of the transistor. Thus, the number of masks can be reduced, sothat manufacturing cost can be reduced.

Next, a third insulating film 10208 is formed and a contact hole isselectively formed in the third insulating film 10208. Note that acontact hole may be formed also in the second insulating film 10204 atthe same time as forming the contact hole in the third insulating film10208.

Next, a third conductive layer 10209 is formed by photolithography, alaser direct writing method, an inkjet method, or the like.

Next, a first alignment film 10210 is formed. Note that after the firstalignment film 10210 is formed, rubbing may be performed so as tocontrol the alignment of the liquid crystal molecules. Rubbing is a stepof forming stripes on an alignment film by rubbing the alignment filmwith a cloth. By performing rubbing, the alignment film can havealignment properties.

The first substrate 10201 which is manufactured as described above andthe second substrate 10216 provided with the light-shielding film 10214,the color filter 10215, the fourth conductive layer 10213, the spacer10217, and the second alignment film 10212 are attached to each other bya sealant with a gap of several μm therebetween. Then, a liquid crystalmaterial is injected into a space between the two substrates. Note thatin the MVA mode, the fourth conductive layer 10213 is formed over theentire surface of the second substrate 10216. Note that the alignmentcontrol protrusion 10219 is formed so as to be in contact with thefourth conductive layer 10213. The alignment control protrusion 10219preferably has a shape with a smooth curved surface. Thus, alignment ofthe adjacent liquid crystal molecules 10218 is extremely similar, sothat an alignment defect can be reduced. Further, a defect of thealignment film caused by disconnection of the alignment film can bereduced.

FIG. 66B is an example of a cross-sectional view of a pixel in the casewhere a PVA (patterned vertical alignment) mode and a transistor arecombined. By applying the pixel structure shown in FIG. 66B to a liquidcrystal display device, a liquid crystal display device having a wideviewing angle, high response speed, and high contrast can be obtained.

Features of the pixel structure shown in FIG. 66B are described below.Liquid crystal molecules 10248 shown in FIG. 66B are long and narrowmolecules each having a major axis and a minor axis. In FIG. 66B, adirection of each of the liquid crystal molecules 10248 is expressed bythe length thereof. That is, the direction of the major axis of theliquid crystal molecule 10248, which is expressed as long, is parallelto the page, and as the liquid crystal molecule 10248 is expressed to beshorter, the direction of the major axis becomes closer to a normaldirection of the page. That is, each of the liquid crystal molecules10248 shown in FIG. 66B is aligned such that the direction of the majoraxis is normal to the alignment film. Thus, the liquid crystal molecules10248 at a position where an electrode notch portion 10249 is formed arealigned radially with a boundary of the electrode notch portion 10249and the fourth conductive layer 10243 as a center. With this state, aliquid crystal display device having a wide viewing angle can beobtained.

Note that the case is described in which a bottom-gate transistor usingan amorphous semiconductor is used as the transistor. In the case wherea transistor using an amorphous semiconductor is used, a liquid crystaldisplay device can be formed at low cost by using a large substrate.

A liquid crystal display device includes a basic portion for displayingimages, which is called a liquid crystal panel. The liquid crystal panelis manufactured as follows: two processed substrates are attached toeach other with a gap of several μm therebetween, and a liquid crystalmaterial is injected into a space between the two substrates. In FIG.66B, the two substrates correspond to a first substrate 10231 and asecond substrate 10246. A transistor and a pixel electrode are formedover the first substrate. A light-shielding film 10244, a color filter10245, a fourth conductive layer 10243, a spacer 10247, and a secondalignment film 10242 are formed over the second substrate.

The light-shielding film 10244 is not necessarily formed over the secondsubstrate 10246. When the light-shielding film 10244 is not formed, thenumber of steps is reduced, so that manufacturing cost can be reduced.Further, since a structure is simple, yield can be improved.Alternatively, when the light-shielding film 10244 is formed, a displaydevice with less light leakage at the time of black display can beobtained.

The color filter 10245 is not necessarily formed over the secondsubstrate 10246. When the color filter 10245 is not formed, the numberof steps is reduced, so that manufacturing cost can be reduced. Further,since a structure is simple, yield can be improved. Note that even whenthe color filter 10245 is not formed, a display device which can performcolor display can be obtained by field sequential driving.Alternatively, when the color filter 10245 is formed, a display devicewhich can perform color display can be obtained.

Spherical spacers may be dispersed over the second substrate 10246instead of forming the spacer 10247. When the spherical spacers aredispersed, the number of steps is reduced, so that manufacturing costcan be reduced. In addition, since a structure is simple when thespherical spacers are dispersed, yield can be improved. Alternatively,when the spacer 10247 is formed, a distance between the two substratescan be uniform because a position of the spacer is not varied, so that adisplay device with little display unevenness can be obtained.

A process to be performed to the first substrate 10231 is describedbelow.

First, a first insulating film 10232 is formed over the first substrate10231 by sputtering, a printing method, a coating method, or the like.Note that the first insulating film 10232 is not necessarily formed. Thefirst insulating film 10232 has a function of preventing change incharacteristics of the transistor due to an adverse effect of animpurity from the first substrate 10231 on a semiconductor layer.

Next, a first conductive layer 10233 is formed over the first insulatingfilm 10232 by photolithography, a laser direct writing method, an inkjetmethod, or the like.

Next, a second insulating film 10234 is formed over the entire surfaceby sputtering, a printing method, a coating method, or the like. Thesecond insulating film 10234 has a function of preventing change incharacteristics of the transistor due to an adverse effect of animpurity from the first substrate 10231 on the semiconductor layer.

Next, a first semiconductor layer 10235 and a second semiconductor layer10236 are formed. Note that the first semiconductor layer 10235 and thesecond semiconductor layer 10236 are formed sequentially and shapesthereof are processed at the same time.

Next, a second conductive layer 10237 is formed by photolithography, alaser direct writing method, an inkjet method, or the like. Note that asa method for etching which is performed at the time of processing ashape of the second conductive layer 10237, dry etching is preferable.Note that as the second conductive layer 10237, either alight-transmitting material or a reflective material may be used.

Next, a channel formation region of the transistor is formed. Here, anexample of a step thereof is described. The second semiconductor layer10236 is etched by using the second conductive layer 10237 as a mask.Alternatively, the second semiconductor layer 10236 is etched by using amask for processing the shape of the second conductive layer 10237.Then, the first conductive layer 10233 at a position where the secondsemiconductor layer 10236 is removed serves as the channel formationregion of the transistor. Thus, the number of masks can be reduced, sothat manufacturing cost can be reduced.

Next, a third insulating film 10238 is formed and a contact hole isselectively formed in the third insulating film 10238. Note that acontact hole may be formed also in the second insulating film 10234 atthe same time as forming the contact hole in the third insulating film10238. Note that a surface of the third insulating film 10238 ispreferably as even as possible. This is because alignment of the liquidcrystal molecules are affected by roughness of a surface with which theliquid crystal is in contact.

Next, a third conductive layer 10239 is formed by photolithography, alaser direct writing method, an inkjet method, or the like.

Next, a first alignment film 10240 is formed. Note that after the firstalignment film 10240 is formed, rubbing may be performed so as tocontrol the alignment of the liquid crystal molecules. Rubbing is a stepof forming stripes on an alignment film by rubbing the alignment filmwith a cloth. By performing rubbing, the alignment film can havealignment properties.

The first substrate 10231 which is manufactured as described above andthe second substrate 10246 provided with the light-shielding film 10244,the color filter 10245, the fourth conductive layer 10243, the spacer10247, and the second alignment film 10242 are attached to each other bya sealant with a gap of several μm therebetween. Then, a liquid crystalmaterial is injected into a space between the two substrates. Note thatin the PVA mode, the fourth conductive layer 10243 is patterned and isprovided with the electrode notch portion 10249. Although a shape of theelectrode notch portion 10249 is not particularly limited, the electrodenotch portion 10249 preferably has a shape in which a plurality ofrectangles having different directions are combined. Thus, a pluralityof regions having different alignment can be formed, so that a liquidcrystal display device having a wide viewing angle can be obtained. Notethat the fourth conductive layer 10243 at the boundary between theelectrode notch portion 10249 and the fourth conductive layer 10243preferably has a shape with a smooth curved surface. Thus, alignment ofthe adjacent liquid crystal molecules 10248 is extremely similar, sothat an alignment defect is reduced. Further, a defect of the alignmentfilm caused by disconnection of the second alignment film 10242 by theelectrode notch portion 10249 can be prevented.

FIG. 67A is an example of a cross-sectional view of a pixel in the casewhere an IPS (in-plane-switching) mode and a transistor are combined. Byapplying the pixel structure shown in FIG. 67A to a liquid crystaldisplay device, a liquid crystal display device theoretically having awide viewing angle and response speed which has low dependency on a grayscale can be obtained.

Features of the pixel structure shown in FIG. 67A are described below.Liquid crystal molecules 10318 shown in FIG. 67A are long and narrowmolecules each having a major axis and a minor axis. In FIG. 67A, adirection of each of the liquid crystal molecules 10318 is expressed bythe length thereof. That is, the direction of the major axis of theliquid crystal molecule 10318, which is expressed as long, is parallelto the page, and as the liquid crystal molecule 10318 is expressed to beshorter, the direction of the major axis becomes closer to a normaldirection of the page. That is, each of the liquid crystal molecules10318 shown in FIG. 67A is aligned so that the direction of the majoraxis thereof is always horizontal to the substrate. Although FIG. 67Ashows alignment with no electric field, when an electric field isapplied to the liquid crystal molecules 10318, each of the liquidcrystal molecules 10318 rotates in a horizontal plane as the directionof the major axis thereof is always horizontal to the substrate. Withthis state, a liquid crystal display device having a wide viewing anglecan be obtained.

Note that the case is described in which a bottom-gate transistor usingan amorphous semiconductor is used as the transistor. In the case wherea transistor using an amorphous semiconductor is used, a liquid crystaldisplay device can be formed at low cost by using a large substrate.

A liquid crystal display device includes a basic portion displayingimages, which is called a liquid crystal panel. The liquid crystal panelis manufactured as follows: two processed substrates are attached toeach other with a gap of several μm therebetween, and a liquid crystalmaterial is injected into a space between the two substrates. In FIG.67A, the two substrates correspond to the first substrate 10301 and thesecond substrate 10316. A transistor and a pixel electrode are formedover the first substrate. A light-shielding film 10314, a color filter10315, a spacer 10317, and a second alignment film 10312 are formed onthe second substrate.

The light-shielding film 10314 is not necessarily formed on the secondsubstrate 10316. When the light-shielding film 10314 is not formed, thenumber of steps is reduced, so that manufacturing cost can be reduced.Further, since a structure is simple, yield can be improved.Alternatively, when the light-shielding film 10314 is formed, a displaydevice with less light leakage at the time of black display can beobtained.

The color filter 10315 is not necessarily formed on the second substrate10316. When the color filter 10315 is not formed, the number of steps isreduced, so that manufacturing cost can be reduced. Further, since astructure is simple, yield can be improved. Note that even when thecolor filter 10315 is not formed, a display device which can performcolor display can be obtained by field sequential driving.Alternatively, when the color filter 10315 is formed, a display devicewhich can perform color display can be obtained.

Spherical spacers may be dispersed on the second substrate 10316 insteadof forming the spacer 10317. When the spherical spacers are dispersed,the number of steps is reduced, so that manufacturing cost can bereduced. In addition, since a structure is simple when the sphericalspacers are dispersed, yield can be improved. Alternatively, when thespacer 10317 is formed, a distance between the two substrates can beuniform because a position of the spacer is not varied, so that adisplay device with little display unevenness can be obtained.

A process to be performed to the first substrate 10301 is described.

First, a first insulating film 10302 is formed over the first substrate10301 by sputtering, a printing method, a coating method, or the like.Note that the first insulating film 10302 is not necessarily formed. Thefirst insulating film 10302 has a function of preventing change incharacteristics of the transistor due to an adverse effect of animpurity from the first substrate 10301 on a semiconductor layer.

Next, a first conductive layer 10303 is formed over the first insulatingfilm 10302 by photolithography, a laser direct writing method, an inkjetmethod, or the like.

Next, a second insulating film 10304 is formed over the entire surfaceby sputtering, a printing method, a coating method, or the like. Thesecond insulating film 10304 has a function of preventing change incharacteristics of the transistor due to an adverse effect of animpurity from the first substrate 10301 on the semiconductor layer.

Next, a first semiconductor layer 10305 and a second semiconductor layer10306 are formed. Note that the first semiconductor layer 10305 and thesecond semiconductor layer 10306 are formed sequentially and shapesthereof are processed at the same time.

Next, a second conductive layer 10307 is formed by photolithography, alaser direct writing method, an inkjet method, or the like. Note that asa method for etching which is performed at the time of processing ashape of the second conductive layer 10307, dry etching is preferable.Note that as the second conductive layer 10307, either alight-transmitting material or a reflective material may be used.

Next, a channel formation region of the transistor is formed. Here, anexample of a step thereof is described. The second semiconductor layer10106 is etched by using the second conductive layer 10307 as a mask.Alternatively, the second semiconductor layer 10306 is etched by using amask for processing the shape of the second conductive layer 10307.Then, the first conductive layer 10303 at a position where the secondsemiconductor layer 10306 is removed serves as the channel formationregion of the transistor. Thus, the number of masks can be reduced, sothat manufacturing cost can be reduced.

Next, a third insulating film 10308 is formed and a contact hole isselectively formed in the third insulating film 10308. Note that acontact hole may be formed also in the second insulating film 10304 atthe same time as forming the contact hole in the third insulating film10308.

Next, a third conductive layer 10309 is formed by photolithography, alaser direct writing method, an inkjet method, or the like. Here, thethird conductive layer 10309 has a shape in which two comb-shapedelectrodes engage with each other. One of the comb-shaped electrodes iselectrically connected to one of a source electrode and a drainelectrode of the transistor, and the other of the comb-shaped electrodesis electrically connected to a common electrode. Thus, a horizontalelectric field can be effectively applied to the liquid crystalmolecules 10318.

Next, a first alignment film 10310 is formed. Note that after the firstalignment film 10310 is formed, rubbing may be performed so as tocontrol the alignment of the liquid crystal molecules. Rubbing is a stepof forming stripes on an alignment film by rubbing the alignment filmwith a cloth. By performing rubbing, the alignment film can havealignment properties.

The first substrate 10301 which is manufactured as described above andthe second substrate 10316 provided with the light-shielding film 10314,the color filter 10315, the spacer 10317, and the second alignment film10312 are formed are attached to each other by a sealant with a gap ofseveral μm therebetween. Then, a liquid crystal material is injectedinto a space between the two substrates.

FIG. 67B is an example of a cross-sectional view of a pixel in the casewhere an FFS (fringe field switching) mode and a transistor arecombined. By applying the pixel structure shown in FIG. 67B to a liquidcrystal display device, a liquid crystal display device theoreticallyhaving a wide viewing angle and response speed which has low dependencyon a gray scale can be obtained.

Features of the pixel structure shown in FIG. 67B are described. Liquidcrystal molecules 10348 shown in FIG. 67B are long and narrow moleculeseach having a major axis and a minor axis. In FIG. 67B, a direction ofeach of the liquid crystal molecules 10348 is expressed by the lengththereof. That is, the direction of the major axis of the liquid crystalmolecule 10348, which is expressed as long, is parallel to the page, andas the liquid crystal molecule 10348 is expressed to be shorter, thedirection of the major axis becomes closer to a normal direction of thepage. That is, each of the liquid crystal molecules 10348 shown in FIG.67B is aligned so that the direction of the major axis thereof is alwayshorizontal to the substrate. Although FIG. 67B shows alignment with noelectric field, when an electric field is applied to the liquid crystalmolecules 10348, each of the liquid crystal molecules 10348 rotates in ahorizontal plane as the direction of the major axis thereof is kept tobe horizontal to the substrate. With this state, a liquid crystaldisplay device having a wide viewing angle can be obtained.

Note that the case is described in which a bottom-gate transistor usingan amorphous semiconductor is used as the transistor. In the case wherea transistor using an amorphous semiconductor is used, a liquid crystaldisplay device can be formed at low cost by using a large substrate.

A liquid crystal display device includes a basic portion for displayingimages, which is called a liquid crystal panel. The liquid crystal panelis manufactured as follows: two processed substrates are attached toeach other with a gap of several μm therebetween, and a liquid crystalmaterial is injected into a space between the two substrates. In FIG.67B, the two substrates correspond to a first substrate 10331 and asecond substrate 10346. A transistor and a pixel electrode are formedover the first substrate. A light-shielding film 10344, a color filter10345, a spacer 10347, and a second alignment film 10342 are formed overthe second substrate.

The light-shielding film 10344 is not necessarily formed over the secondsubstrate 10346. When the light-shielding film 10344 is not formed, thenumber of steps is reduced, so that manufacturing cost can be reduced.Further, since a structure is simple, yield can be improved.Alternatively, when the light-shielding film 10344 is formed, a displaydevice with less light leakage at the time of black display can beobtained.

The color filter 10345 is not necessarily formed over the secondsubstrate w10346. When the color filter 10345 is not formed, the numberof steps is reduced, so that manufacturing cost can be reduced. Further,since a structure is simple, yield can be improved. Note that even whenthe color filter 10345 is not formed, a display device which can performcolor display can be obtained by field sequential driving.Alternatively, when the color filter 10345 is formed, a display devicewhich can perform color display can be obtained.

Spherical spacers may be dispersed over the second substrate 10346instead of forming the spacer 10347. When the spherical spacers aredispersed, the number of steps is reduced, so that manufacturing costcan be reduced. In addition, since a structure is simple when thespherical spacers are dispersed, yield can be improved. Alternatively,when the spacer 10347 is formed, a distance between the two substratescan be uniform because a position of the spacer is not varied, so that adisplay device with little display unevenness can be obtained.

A process to be performed to the first substrate 10331 is describedbelow.

First, a first insulating film 10332 is formed over the first substrate10331 by sputtering, a printing method, a coating method, or the like.Note that the first insulating film 10332 is not necessarily formed. Thefirst insulating film 10332 has a function of preventing change incharacteristics of the transistor due to an adverse effect of animpurity from the first substrate 10331 on a semiconductor layer.

Next, a first conductive layer 10333 is formed over the first insulatingfilm 10332 by photolithography, a laser direct writing method, an inkjetmethod, or the like.

Next, a second insulating film 10334 is formed over the entire surfaceby sputtering, a printing method, a coating method, or the like. Thesecond insulating film 10334 has a function of preventing change incharacteristics of the transistor due to an adverse effect of animpurity from the first substrate 10331 on the semiconductor layer.

Next, a first semiconductor layer 10335 and a second semiconductor layer10336 are formed. Note that the first semiconductor layer 10335 and thesecond semiconductor layer 10336 are formed sequentially and shapesthereof are processed at the same time.

Next, a second conductive layer 10337 is formed by photolithography, alaser direct writing method, an inkjet method, or the like. Note that asa method for etching which is performed at the time of processing ashape of the second conductive layer 10337, dry etching is preferable.Note that as the second conductive layer 10337, either alight-transmitting material or a reflective material may be used.

Next, a channel formation region of the transistor is formed. Here, anexample of a step thereof is described. The second semiconductor layer10336 is etched by using the second conductive layer 10337 as a mask.Alternatively, the second semiconductor layer 10336 is etched by using amask for processing the shape of the second conductive layer 10337.Then, the first conductive layer 10333 at a position where the secondsemiconductor layer 10336 is removed serves as the channel formationregion of the transistor. Thus, the number of masks can be reduced, sothat manufacturing cost can be reduced.

Next, a third insulating film 10338 is formed and a contact hole isselectively formed in the third insulating film 10338.

Next, a fourth conductive layer 10343 is formed by photolithography, alaser direct writing method, an inkjet method, or the like.

Next, a fourth insulating film 10349 is formed and a contact hole isselectively formed in the fourth insulating film 10349. Note that asurface of the fourth insulating film 10349 is preferably as even aspossible. This is because alignment of the liquid crystal molecules areaffected by roughness of a surface with which the liquid crystal is incontact.

Next, a third conductive layer 10339 is formed by photolithography, alaser direct writing method, an inkjet method, or the like. Here, thethird conductive layer 10339 is comb-shaped.

Next, a first alignment film 10340 is formed. Note that after the firstalignment film 10340 is formed, rubbing may be performed so as tocontrol the alignment of the liquid crystal molecules. Rubbing is a stepof forming stripes on an alignment film by rubbing the alignment filmwith a cloth. By performing rubbing, the alignment film can havealignment properties.

The first substrate 10331 which is manufactured as described above andthe second substrate 10346 provided with the light-shielding film 10344,the color filter 10345, the spacer 10347, and the second alignment film10342 are attached to each other by a sealant with a gap of several μmtherebetween. Then, a liquid crystal material is injected into a spacebetween the two substrates. Accordingly, a liquid crystal panel can bemanufactured.

Here, materials which can be used for conductive layers or insulatingfilms are described.

As the first insulating film 10102 in FIG. 65, the first insulating film10202 in FIG. 66A, the first insulating film 10232 in FIG. 66B, thefirst insulating film 10302 in FIG. 67A, or the first insulating film10332 in FIG. 67B, an insulating film such as a silicon oxide film, asilicon nitride film, or a silicon oxynitride (SiO_(x)N_(y)) film can beused. Alternatively, an insulating film having a stacked-layer structurein which two or more of a silicon oxide film, a silicon nitride film, asilicon oxynitride (SiO_(x)N_(y)) film, and the like are combined can beused as.

As the first conductive layer 10103 in FIG. 65, the first conductivelayer 10203 in FIG. 66A, the first conductive layer 10233 in FIG. 66B,the first conductive layer 10303 in FIG. 67A, or the first conductivelayer 10333 in FIG. 67B, Mo, Ti, Al, Nd, Cr, or the like can be used.Alternatively, a stacked-layer structure in which two or more of Mo, Ti,Al, Nd, Cr, and/or the like are combined can be used.

As the second insulating film 10104 in FIG. 65, the second insulatingfilm 10204 in FIG. 66A, the second insulating film 10234 in FIG. 66B,the second insulating film 10304 in FIG. 67A, or the second insulatingfilm 10334 in FIG. 67B, a thermal oxide film, a silicon oxide film, asilicon nitride film, a silicon oxynitride film, or the like can beused. Alternatively, a stacked-layer structure in which two or more of athermal oxide film, a silicon oxide film, a silicon nitride film, asilicon oxynitride film, and/or the like are combined can be used. Notethat a silicon oxide film is preferable in a portion which is in contactwith a semiconductor layer. This is because a trap level at an interfacewith the semiconductor layer decreases when a silicon oxide film isused. Note that a silicon nitride film is preferable in a portion whichis in contact with Mo. This is because a silicon nitride film does notoxidize Mo.

As the first semiconductor layer 10105 in FIG. 65, the firstsemiconductor layer 10205 in FIG. 66A, the first semiconductor layer10235 in FIG. 66B, the first semiconductor layer 10305 in FIG. 67A, orthe first semiconductor layer 10335 in FIG. 67B, silicon, silicongermanium (SiGe), or the like can be used.

As the second semiconductor layer 10106 in FIG. 65, the secondsemiconductor layer 10206 in FIG. 66A, the second semiconductor layer10236 in FIG. 66B, the second semiconductor layer 10306 in FIG. 67A, orthe second semiconductor layer 10336 in FIG. 67B, silicon or the likeincluding phosphorus can be used, for example.

As a light-transmitting material of the second conductive layer 10107and the third conductive layer 10109 in FIG. 65; the second conductivelayer 10207 and the third conductive layer 10209 in FIG. 66A; the secondconductive layer 10237 and the third conductive layer 10239 in FIG. 66B;the second conductive layer 10307 and the third conductive layer 10309in FIG. 67A; or the second conductive layer 10337, the third conductivelayer 10339, the fourth conductive layer 10113 in FIG. 65, the fourthconductive layer 10213 in FIG. 66A, the fourth conductive layer 10243 inFIG. 66B, and the fourth conductive layer 10343 in FIG. 67B, an indiumtin oxide (ITO) film formed by mixing tin oxide into indium oxide, anindium tin silicon oxide (ITSO) film formed by mixing silicon oxide intoindium tin oxide (ITO), an indium zinc oxide (IZO) film formed by mixingzinc oxide into indium oxide, a zinc oxide film, a tin oxide film, orthe like can be used. Note that IZO is a light-transmitting conductivematerial formed by sputtering using a target in which zinc oxide (ZnO)is mixed into ITO at 2 to 20 wt %.

As a reflective material of the second conductive layer 10107 and thethird conductive layer 10109 in FIG. 65; the second conductive layer10207 and the third conductive layer 10209 in FIG. 66A; the secondconductive layer 10237 and the third conductive layer 10239 in FIG. 66B;the second conductive layer 10307 and the third conductive layer 10309in FIG. 67A; or the second conductive layer 10337, the third conductivelayer 10339, and the fourth conductive layer 10343 in FIG. 67B, Ti, Mo,Ta, Cr, W, Al, or the like can be used. Alternatively, a two-layerstructure in which Al and Ti, Mo, Ta, Cr, or W are stacked, or athree-layer structure in which Al is interposed between metals such asTi, Mo, Ta, Cr, and W may be used.

As the third insulating film 10108 in FIG. 65, the third insulating film10208 in FIG. 66A, the third insulating film 10238 in FIG. 66B, thethird insulating film 10308 in FIG. 67A, or the third insulating film10338 and the fourth insulating film 10349 in FIG. 67B, an inorganicmaterial (e.g., silicon oxide, silicon nitride, or silicon oxynitride),an organic compound material having a low dielectric constant (e.g., aphotosensitive or nonphotosensitive organic resin material), or the likecan be used. Alternatively, a material including siloxane can be used.Note that siloxane is a material in which a skeleton structure is formedby a bond of silicon (Si) and oxygen (O). As a substituent, an organicgroup including at least hydrogen (e.g., an alkyl group or aromatichydrocarbon) is used. Alternatively, a fluoro group may be used as thesubstituent. Further alternatively, an organic group including at leasthydrogen and a fluoro group may be used as the substituent.

As the first alignment film 10110 in FIG. 65, the first alignment film10210 in FIG. 66A, the first alignment film 10240 in FIG. 66B, the firstalignment film 10310 in FIG. 67A, or the first alignment film 10340 inFIG. 67B, a film of a polymer such as polyimide can be used.

Next, the pixel structure in the case where each liquid crystal mode andthe transistor are combined is described with reference to a top planview (a layout diagram) of the pixel.

Note that as a liquid crystal mode, a TN (twisted nematic) mode, an IPS(in-plane-switching) mode, an FFS (fringe field switching) mode, an MVA(multi-domain vertical alignment) mode, a PVA (patterned verticalalignment) mode, an ASM (axially symmetric aligned micro-cell) mode, anOCB (optical compensated birefringence) mode, an FLC (ferroelectricliquid crystal) mode, an AFLC (antiferroelectric liquid crystal) mode,or the like can be used.

As the transistor, a thin film transistor (a TFT) including a non-singlecrystalline semiconductor layer typified by an amorphous silicon layer,a polycrystalline silicon layer, a microcrystalline (also referred to assemi-amorphous) silicon layer, or the like can be used.

Note that as a structure of the transistor, a top-gate structure, abottom-gate structure, or the like can be used. A channel-etchedtransistor, a channel-protective transistor, or the like can be used asa bottom-gate transistor.

FIG. 68 is an example of a top plan view of a pixel in the case where aTN mode and a transistor are combined. By applying the pixel structureshown in FIG. 68 to a liquid crystal display device, a liquid crystaldisplay device can be formed at low cost.

The pixel shown in FIG. 68 includes a scan line 10401, a video signalline 10402, a capacitor line 10403, a transistor 10404, a pixelelectrode 10405, and a pixel capacitor 10406.

The scan line 10401 has a function of transmitting a signal (a scansignal) to the pixel. The video signal line 10402 has a function oftransmitting a signal (an image signal) to the pixel. Note that sincethe scan lines 10401 and the video signal lines 10402 are arranged inmatrix, they are formed of conductive layers in different layers. Notethat a semiconductor layer may be provided at an intersection of thescan line 10401 and the video signal line 10402; thus, intersectioncapacitance formed between the scan line 10401 and the video signal line10402 can be reduced.

The capacitor line 10403 is provided in parallel to the pixel electrode10405. A portion where the capacitor line 10403 and the pixel electrode10405 overlap with each other corresponds to the pixel capacitor 10406.Note that part of the capacitor line 10403 is extended along the videosignal line 10402 so as to surround the video signal line 10402. Thus,crosstalk can be reduced. Crosstalk is a phenomenon in which a potentialof an electrode, which is needed to be kept, is changed in accordancewith change in potential of the video signal line 10402. Note thatintersection capacitance can be reduced by providing a semiconductorlayer between the capacitor line 10403 and the video signal line 10402.Note that the capacitor line 10403 is formed of a material which issimilar to that of the scan line 10401.

The transistor 10404 has a function as a switch which electricallyconnects the video signal line 10402 and the pixel electrode 10405. Notethat one of a source region and a drain region of the transistor 10404is provided so as to be surrounded by the other of the source region andthe drain region of the transistor 10404. Thus, the channel width of thetransistor 10404 increases, so that switching capability can beimproved. Note that a gate electrode of the transistor 10404 is providedso as to surround the semiconductor layer.

The pixel electrode 10405 is electrically connected to one of a sourceelectrode and a drain electrode of the transistor 10404. The pixelelectrode 10405 is an electrode for applying signal voltage which istransmitted by the video signal line 10402 to a liquid crystal element.Note that the pixel electrode 10405 is rectangular. Thus, an apertureratio of the pixel can be improved. Note that as the pixel electrode10405, a light-transmitting material or a reflective material may beused. Alternatively, the pixel electrode 10405 may be formed bycombining a light-transmitting material and a reflective material.

FIG. 69A is an example of a top plan view of a pixel in the case wherean MVA mode and a transistor are combined. By applying the pixelstructure shown in FIG. 69A to a liquid crystal display device, a liquidcrystal display device having a wide viewing angle, high response speed,and high contrast can be obtained.

The pixel shown in FIG. 69A includes a scan line 10501, a video signalline 10502, a capacitor line 10503, a transistor 10504, a pixelelectrode 10505, a pixel capacitor 10506, and an alignment controlprotrusion 10507.

The scan line 10501 has a function of transmitting a signal (a scansignal) to the pixel. The video signal line 10502 has a function oftransmitting a signal (an image signal) to the pixel. Note that sincethe scan lines 10501 and the video signal lines 10502 are arranged inmatrix, they are formed of conductive layers in different layers. Notethat a semiconductor layer may be provided at an intersection of thescan line 10501 and the video signal line 10502; thus, intersectioncapacitance formed between the scan line 10501 and the video signal line10502 can be reduced.

The capacitor line 10503 is provided in parallel to the pixel electrode10505. A portion where the capacitor line 10503 and the pixel electrode10505 overlap with each other corresponds to the pixel capacitor 10506.Note that part of the capacitor line 10503 is extended along the videosignal line 10502 so as to surround the video signal line 10502. Thus,crosstalk can be reduced. Crosstalk is a phenomenon in which a potentialof an electrode, which is needed to be kept, is changed in accordancewith change in potential of the video signal line 10502. Note thatintersection capacitance can be reduced by providing a semiconductorlayer between the capacitor line 10503 and the video signal line 10502.Note that the capacitor line 10503 is formed of a material which issimilar to that of the scan line 10501.

The transistor 10504 has a function as a switch which electricallyconnects the video signal line 10502 and the pixel electrode 10505. Notethat one of a source region and a drain region of the transistor 10504is provided so as to be surrounded by the other of the source region andthe drain region of the transistor 10504. Thus, the channel width of thetransistor 10504 increases, so that switching capability can beimproved. Note that a gate electrode of the transistor 10504 is providedso as to surround the semiconductor layer.

The pixel electrode 10505 is electrically connected to one of a sourceelectrode and a drain electrode of the transistor 10504. The pixelelectrode 10505 is an electrode for applying signal voltage which istransmitted by the video signal line 10502 to a liquid crystal element.Note that the pixel electrode 10505 is rectangular. Thus, an apertureratio of the pixel can be improved. Note that as the pixel electrode10505, a light-transmitting material or a reflective material may beused. Alternatively, the pixel electrode 10505 may be formed bycombining a light-transmitting material and a reflective material.

The alignment control protrusion 10507 is formed over a countersubstrate. The alignment control protrusion 10507 has a function ofaligning liquid crystal molecules radially. Note that a shape of thealignment control protrusion 10507 is not particularly limited. Forexample, the alignment control protrusion 10507 may be a dogleg shape.Thus, a plurality of regions having different alignment of the liquidcrystal molecules can be formed, so that a viewing angle can beimproved.

FIG. 69B is an example of a top plan view of a pixel in the case where aPVA mode and a transistor are combined. By applying the pixel structureshown in FIG. 69B to a liquid crystal display device, a liquid crystaldisplay device having a wide viewing angle, high response speed, andhigh contrast can be obtained.

The pixel shown in FIG. 69B includes a scan line 10511, a video signalline 10512, a capacitor line 10513, a transistor 10514, a pixelelectrode 10515, a pixel capacitor 10516, and an electrode notch portion10517.

The scan line 10511 has a function of transmitting a signal (a scansignal) to the pixel. The video signal line 10512 has a function oftransmitting a signal (an image signal) to the pixel. Note that sincethe scan lines 10511 and the video signal lines 10512 are arranged inmatrix, they are formed of conductive layers in different layers. Notethat a semiconductor layer may be provided at an intersection of thescan line 10511 and the video signal line 10512; thus, intersectioncapacitance formed between the scan line 10511 and the video signal line10512 can be reduced.

The capacitor line 10513 is provided in parallel to the pixel electrode10515. A portion where the capacitor line 10513 and the pixel electrode10515 overlap with each other corresponds to the pixel capacitor 10516.Note that part of the capacitor line 10513 is extended along the videosignal line 10512 so as to surround the video signal line 10512. Thus,crosstalk can be reduced. Crosstalk is a phenomenon in which a potentialof an electrode, which is needed to be kept, is changed in accordancewith change in potential of the video signal line 10512. Note thatintersection capacitance can be reduced by providing a semiconductorlayer between the capacitor line 10513 and the video signal line 10512.Note that the capacitor line 10513 is formed of a material which issimilar to that of the scan line 10511.

The transistor 10514 has a function as a switch which electricallyconnects the video signal line 10512 and the pixel electrode 10515. Notethat one of a source region and a drain region of the transistor 10514is provided so as to be surrounded by the other of the source region andthe drain region of the transistor 10514. Thus, the channel width of thetransistor 10514 increases, so that switching capability can beimproved. Note that a gate electrode of the transistor 10514 is providedso as to surround the semiconductor layer.

The pixel electrode 10515 is electrically connected to one of a sourceelectrode and a drain electrode of the transistor 10514. The pixelelectrode 10515 is an electrode for applying signal voltage which istransmitted by the video signal line 10512 to a liquid crystal element.Note that the pixel electrode 10515 has a shape which is formed inaccordance with a shape of the electrode notch portion 10517.Specifically, the pixel electrode 10515 has a shape in which a portionwhere the pixel electrode 10515 is notched is formed in a portion wherethe electrode notch portion 10517 is not formed. Thus, a plurality ofregions having different alignment of the liquid crystal molecules canbe formed, so that a viewing angle can be improved. Note that as thepixel electrode 10515, a light-transmitting material or a reflectivematerial may be used. Alternatively, the pixel electrode 10515 may beformed by combining a light-transmitting material and a reflectivematerial.

FIG. 70A is an example of a top plan view of a pixel in the case wherean IPS mode and a transistor are combined. By applying the pixelstructure shown in FIG. 70A to a liquid crystal display device, a liquidcrystal display device theoretically having a wide viewing angle andresponse speed which has low dependency on a gray scale can be obtained.

The pixel shown in FIG. 70A includes a scan line 10601, a video signalline 10602, a common electrode 10603, a transistor 10604, and a pixelelectrode 10605.

The scan line 10601 has a function of transmitting a signal (a scansignal) to the pixel. The video signal line 10602 has a function oftransmitting a signal (an image signal) to the pixel. Note that sincethe scan lines 10601 and the video signal lines 10602 are arranged inmatrix, they are formed of conductive layers in different layers. Notethat a semiconductor layer may be provided at an intersection of thescan line 10601 and the video signal line 10602; thus, intersectioncapacitance formed between the scan line 10601 and the video signal line10602 can be reduced. Note that the video signal line 10602 is formed inaccordance with a shape of the pixel electrode 10605.

The common electrode 10603 is provided in parallel to the pixelelectrode 10605. The common electrode 10603 is an electrode forgenerating a horizontal electric field. Note that the common electrode10603 is bent comb-shaped. Note that part of the common electrode 10603is extended along the video signal line 10602 so as to surround thevideo signal line 10602. Thus, crosstalk can be reduced. Crosstalk is aphenomenon in which a potential of an electrode, which is needed to bekept, is changed in accordance with change in potential of the videosignal line 10602. Note that intersection capacitance can be reduced byproviding a semiconductor layer between the common electrode 10603 andthe video signal line 10602. Part of the common electrode 10603, whichis provided in parallel to the scan line 10601, is formed of a materialwhich is similar to that of the scan line 10601. Part of the commonelectrode 10603, which is provided in parallel to the pixel electrode10605, is formed of a material which is similar to that of the pixelelectrode 10605.

The transistor 10604 has a function as a switch which electricallyconnects the video signal line 10602 and the pixel electrode 10605. Notethat one of a source region and a drain region of the transistor 10604is provided so as to be surrounded by the other of the source region andthe drain region of the transistor 10604. Thus, the channel width of thetransistor 10604 increases, so that switching capability can beimproved. Note that a gate electrode of the transistor 10604 is providedso as to surround the semiconductor layer.

The pixel electrode 10605 is electrically connected to one of a sourceelectrode and a drain electrode of the transistor 10604. The pixelelectrode 10605 is an electrode for applying signal voltage which istransmitted by the video signal line 10602 to a liquid crystal element.Note that the pixel electrode 10605 is bent comb-shaped. Thus, ahorizontal electric field can be applied to liquid crystal molecules. Inaddition, since a plurality of regions having different alignment of theliquid crystal molecules can be formed, a viewing angle can be improved.Note that as the pixel electrode 10605, a light-transmitting material ora reflective material may be used. Alternatively, the pixel electrode10605 may be formed by combining a light-transmitting material and areflective material.

Note that a comb-shaped portion in the common electrode 10603 and thepixel electrode 10605 may be formed of different conductive layers. Forexample, the comb-shaped portion in the common electrode 10603 may beformed of the same conductive layer as the scan line 10601 or the videosignal line 10602. Similarly, the pixel electrode 10605 may be formed ofthe same conductive layer as the scan line 10601 or the video signalline 10602.

FIG. 70B is a top plan view of a pixel in the case where an FFS mode anda transistor are combined. By applying the pixel structure shown in FIG.70B to a liquid crystal display device, a liquid crystal display devicetheoretically having a wide viewing angle and response speed which haslow dependency on a gray scale can be obtained.

The pixel shown in FIG. 70B may include a scan line 10611, a videosignal line 10612, a common electrode 10613, a transistor 10614, and apixel electrode 10615.

The scan line 10611 has a function of transmitting a signal (a scansignal) to the pixel. The video signal line 10612 has a function oftransmitting a signal (an image signal) to the pixel. Note that sincethe scan lines 10611 and the video signal lines 10612 are arranged inmatrix, they are formed of conductive layers in different layers. Notethat a semiconductor layer may be provided at an intersection of thescan line 10611 and the video signal line 10612; thus, intersectioncapacitance formed between the scan line 10611 and the video signal line10612 can be reduced. Note that the video signal line 10612 is formed inaccordance with a shape of the pixel electrode 10615.

The common electrode 10613 is formed uniformly below the pixel electrode10615 and below and between the pixel electrodes 10615. Note that as thecommon electrode 10613, either a light-transmitting material or areflective material may be used. Alternatively, the common electrode10613 may be formed by combining a material in which alight-transmitting material and a reflective material.

The transistor 10614 has a function as a switch which electricallyconnects the video signal line 10612 and the pixel electrode 10615. Notethat one of a source region and a drain region of the transistor 10614is provided so as to be surrounded by the other of the source region andthe drain region of the transistor 10614. Thus, the channel width of thetransistor 10614 increases, so that switching capability can beimproved. Note that a gate electrode of the transistor 10614 is providedso as to surround the semiconductor layer.

The pixel electrode 10615 is electrically connected to one of a sourceelectrode and a drain electrode of the transistor 10614. The pixelelectrode 10615 is an electrode for applying signal voltage which istransmitted by the video signal line 10612 to a liquid crystal element.Note that the pixel electrode 10615 is bent comb-shaped. The comb-shapedpixel electrode 10615 is provided to be closer to a liquid crystal layerthan a uniform portion of the common electrode 10613. Thus, a horizontalelectric field can be applied to liquid crystal molecules. In addition,a plurality of regions having different alignment of the liquid crystalmolecules can be formed, so that a viewing angle can be improved. Notethat as the pixel electrode 10615, a light-transmitting material or areflective material may be used. Alternatively, the pixel electrode10615 may be formed by combining a light-transmitting material and areflective material.

Note that although this embodiment mode is described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, even more drawings can be formed by combining each part withanother part in the above-described drawings.

Similarly, the contents (or part of the contents) described in eachdrawing of this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, even more drawings can beformed by combining each part with part of another embodiment mode inthe drawings of this embodiment mode.

This embodiment mode shows an example of an embodied case of thecontents (or part of the contents) described in another embodiment mode,an example of slight transformation thereof, an example of partialmodification thereof, an example of improvement thereof, an example ofdetailed description thereof, an application example thereof, an exampleof related part thereof, or the like. Therefore, the contents describedin another embodiment mode can be freely applied to, combined with, orreplaced with this embodiment mode.

Embodiment Mode 14

In this embodiment mode, steps of manufacturing a liquid crystal cell(also referred to as a liquid crystal panel) are described.

Steps of manufacturing a liquid crystal cell in the case where a vacuuminjection method is used as a method of filling with liquid crystals aredescribed with reference to FIGS. 71A to 71E and 72A to 72C.

FIG. 72C is a cross-sectional view of a liquid crystal cell. A firstsubstrate 70101 and a second substrate 70107 are attached with spacers70106 and a sealing material 70105 interposed therebetween. Liquidcrystals 70109 are arranged between the first substrate 70101 and thesecond substrate 70107. Note that an alignment film 70102 is formed overthe first substrate 70101, and an alignment film 70108 is formed overthe second substrate 70107.

The first substrate 70101 is provided with a plurality of pixelsarranged in matrix. Each of the plurality of pixels may include atransistor. Note that the first substrate 70101 may be referred to as aTFT substrate, an array substrate, or a TFT array substrate. Examples ofthe first substrate 70101 include, but are not limited to, a singlecrystalline substrate, an SOI substrate, a glass substrate, a quartzsubstrate, a plastic substrate, a paper substrate, a cellophanesubstrate, a stone substrate, a wood substrate, a cloth substrate(including a natural fiber (e.g., silk, cotton, or hemp), a syntheticfiber (e.g., nylon, polyurethane, or polyester), and a regenerated fiber(e.g., acetate, cupra, rayon, or regenerated polyester)), a leathersubstrate, a rubber substrate, a stainless steel substrate, and asubstrate including stainless steel foil. Alternatively, a skin (e.g.,epidermis or corium) or hypodermal tissue of an animal such as a humancan be used as the substrate.

The second substrate 70107 is provided with a common electrode, a colorfilter, a black matrix, and the like. Note that the second substrate70107 may be referred to as a counter substrate or a color filtersubstrate.

The alignment film 70102 has a function of aligning liquid crystalmolecules in a certain direction. An example of the alignment film 70102includes, but is not limited to, a polyimide resin. Note that thealignment film 70108 is similar to the alignment film 70102.

The sealing material 70105 has a function of bonding the first substrate70101 and the second substrate 70107 so that the liquid crystals 70109are not leaked. That is, the sealing material 70105 functions as asealant.

The spacer 70106 has a function of maintaining a fixed space between thefirst substrate 70101 and the second substrate 70107 (a cell gap of theliquid crystal). As the spacer 70106, a granular spacer or a columnarspacer can be used. Examples of the granular spacer include afiber-shaped spacer and a spherical spacer. Examples of a material forthe granular spacer include plastic and glass. A spherical spacer formedof plastic is called a plastic bead and has been widely used. Afiber-shaped spacer formed of glass is called a glass fiber and mixed ina sealing material when used.

FIG. 71A is a cross-sectional view of a step of forming the alignmentfilm 70102 over the first substrate 70101. The alignment film 70102 isformed over the first substrate 70101 by a roller coating method withthe use of a roller 70103. Note that other than a roller coating method,an offset printing method, a dip coating method, an air-knife method, acurtain coating method, a wire-bar coating method, a gravure coatingmethod, an extrusion coating method, or the like can be used.Thereafter, pre-baking and main-baking are sequentially performed on thealignment film 70102.

FIG. 71B is a cross-sectional view of a step of performing rubbingtreatment on the alignment film 70102. The rubbing treatment isperformed by rotating a roller 70104 for rubbing, in which a cloth iswrapped around a drum, to rub the alignment film 70102. When the rubbingtreatment is performed on the alignment film 70102, a groove foraligning liquid crystal molecules in a certain direction is formed inthe alignment film 70102. Note that the invention is not limited to thisstructure, and a groove may be formed in the alignment film by using anion beam. Thereafter, water washing treatment is performed on the firstsubstrate 70101. Accordingly, contaminant, dirt, or the like on asurface of the first substrate 70101 can be removed.

Although not shown, similarly to the first substrate 70101, thealignment film 70108 is formed over the second substrate 70107, andrubbing treatment is performed on the alignment film 70108. Note thatthe invention is not limited to this structure, and a groove may beformed in the alignment film by using an ion beam.

FIG. 71C is a cross-sectional view of a step of forming the sealingmaterial 70105 over the alignment film 70102. The sealing material 70105is applied by a lithography device, screen printing, or the like, and aseal pattern is formed. The seal pattern is formed along the peripheryof the first substrate 70101, and a liquid crystal inlet is provided inpart of the seal pattern. A UV resin for temporal fixing is spot-appliedto a region other than a display region of the first substrate 70101 bya dispenser or the like.

Note that the sealing material 70105 may be provided for the secondsubstrate 70107.

FIG. 71D is a cross-sectional view of a step of dispersing the spacers70106 over the first substrate 70101. The spacers 70106 are ejected by anozzle together with a compressed gas and dispersed (dry dispersion).Alternatively, the spacers 70106 are mixed in a volatile liquid, and theliquid is sprayed so as to be dispersed (wet dispersion). By such drydispersion or wet dispersion, the spacers 70106 can be uniformlydispersed over the first substrate 70101.

In this embodiment mode, the case where the spherical spacer of thegranular spacer is used as the spacer 70106 is described; however, theinvention is not limited thereto, and a columnar spacer may be used. Thecolumnar spacer may be provided for the first substrate 70101 or thesecond substrate 70107. Alternatively, a part of the spacers may beprovided for the first substrate 70101 and the other part thereof may beprovided for the second substrate 70107.

Note that a spacer may be mixed in the sealing material. Accordingly,the cell gap of the liquid crystal can be maintained constant moreeasily.

FIG. 71E is a cross-sectional view of a step of attaching the firstsubstrate 70101 and the second substrate 70107. The first substrate70101 and the second substrate 70107 are attached in the atmosphere.Then, the first substrate 70101 and the second substrate 70107 arepressurized so that a gap between the first substrate 70101 and thesecond substrate 70107 is constant. Thereafter, ultraviolet rayirradiation or heat treatment is performed on the sealing material70105; thus, the sealing material 70105 is hardened.

FIGS. 72A and 72B are top plan views of steps of filling with liquidcrystals. A cell in which the first substrate 70101 and the secondsubstrate 70107 are attached (also referred to as an empty cell) isplaced in a vacuum chamber. Then, the pressure in the vacuum chamber isreduced, and thereafter, a liquid crystal inlet 70113 of the empty cellis immersed in liquid crystals. Then, when the vacuum chamber is openedto the atmosphere, the empty cell is filled with the liquid crystals70109 due to pressure difference and a capillary phenomenon.

When the empty cell is filled with the needed amount of liquid crystals70109, the liquid crystal inlet is sealed by a resin 70110. Then, extraliquid crystals attached to the empty cell are washed out. Thereafter,realignment treatment is performed on the liquid crystals 70109 byannealing treatment. Accordingly, the liquid crystal cell is completed.

Next, steps of manufacturing a liquid crystal cell in the case where adropping method is used as a method of filling with liquid crystals aredescribed with reference to FIGS. 73A to 73D and 74A to 74C.

FIG. 74C is a cross-sectional view of a liquid crystal cell. A firstsubstrate 70301 and a second substrate 70307 are attached with spacers70306 and a sealing material 70305 interposed therebetween. Liquidcrystals 70309 are arranged between the first substrate 70301 and thesecond substrate 70307. Note that an alignment film 70302 is formed overthe first substrate 70301, and an alignment film 70308 is formed overthe second substrate 70307.

The first substrate 70301 is provided with a plurality of pixelsarranged in matrix. Each of the plurality of pixels may include atransistor. Note that the first substrate 70301 may be referred to as aTFT substrate, an array substrate, or a TFT array substrate. Examples ofthe first substrate 70301 include, but are not limited to, a singlecrystalline substrate, an SOI substrate, a glass substrate, a quartzsubstrate, a plastic substrate, a paper substrate, a cellophanesubstrate, a stone substrate, a wood substrate, a cloth substrate(including a natural fiber (e.g., silk, cotton, or hemp), a syntheticfiber (e.g., nylon, polyurethane, or polyester), and a regenerated fiber(e.g., acetate, cupra, rayon, or regenerated polyester)), a leathersubstrate, a rubber substrate, a stainless steel substrate, and asubstrate including stainless steel foil. Alternatively, a skin (e.g.,epidermis or corium) or hypodermal tissue of an animal such as a humancan be used as the substrate.

The second substrate 70307 is provided with a common electrode, a colorfilter, a black matrix, and the like. Note that the second substrate70307 may be referred to as a counter substrate or a color filtersubstrate.

The alignment film 70302 has a function of aligning liquid crystalmolecules in a certain direction. An example of the alignment film 70302includes, but is not limited to, a polyimide resin. Note that thealignment film 70308 is similar to the alignment film 70302.

The sealing material 70305 has a function of bonding the first substrate70301 and the second substrate 70307 so that the liquid crystals 70309are not leaked. That is, the sealing material 70305 functions as asealant.

The spacer 70306 has a function of maintaining a fixed space between thefirst substrate 70301 and the second substrate 70307 (a cell gap of theliquid crystal). As the spacer 70306, a granular spacer or a columnarspacer can be used. Examples of the granular spacer include afiber-shaped spacer and a spherical spacer. Examples of a material forthe granular spacer include plastic and glass. A spherical spacer formedof plastic is called a plastic bead and has been widely used. Afiber-shaped spacer formed of glass is called a glass fiber and mixed ina sealing material when used.

FIG. 73A is a cross-sectional view of a step of forming the alignmentfilm 70302 over the first substrate 70301. The alignment film 70302 isformed over the first substrate 70301 by a roller coating method withthe use of a roller 70303. Note that other than a roller coating method,an offset printing method, a dip coating method, an air-knife method, acurtain coating method, a wire-bar coating method, a gravure coatingmethod, an extrusion coating method, or the like can be used.Thereafter, pre-baking and main-baking are sequentially performed on thealignment film 70302.

FIG. 73B is a cross-sectional view of a step of performing rubbingtreatment on the alignment film 70302. The rubbing treatment isperformed by rotating a roller 70304 for rubbing, in which a cloth iswrapped around a drum, to rub the alignment film 70302. When the rubbingtreatment is performed on the alignment film 70302, a groove foraligning liquid crystal molecules in a certain direction is formed inthe alignment film 70302. Note that the invention is not limited to thisstructure, and a groove may be formed in the alignment film by using anion beam. Thereafter, water washing treatment is performed on the firstsubstrate 70301. Accordingly, contaminant, dirt, or the like on asurface of the first substrate 70301 can be removed.

Although not shown, similarly to the first substrate 70301, thealignment film 70308 is formed over the second substrate 70307, andrubbing treatment is performed on the alignment film 70308. Note thatthe invention is not limited to this structure, and a groove may beformed in the alignment film by using an ion beam.

FIG. 73C is a cross-sectional view of a step of forming the sealingmaterial 70305 over the alignment film 70302. The sealing material 70305is applied by a lithography device, screen printing, or the like, and aseal pattern is formed. The seal pattern is formed along the peripheryof the first substrate 70301. In this embodiment mode, a radical UVresin or a cationic UV resin is used for the sealing material 70305.Then, a conductive resin is spot-applied by a dispenser.

Note that the sealing material 70305 may be provided for the secondsubstrate 70307.

FIG. 73D is a cross-sectional view of a step of dispersing the spacers70306 over the first substrate 70301. The spacers 70306 are ejected by anozzle together with a compressed gas and dispersed (dry dispersion).Alternatively, the spacers 70306 are mixed in a volatile liquid, and theliquid is sprayed so as to be dispersed (wet dispersion). By such drydispersion or wet dispersion, the spacer 70306 can be uniformlydispersed over the first substrate 70301.

In this embodiment mode, the case where the spherical spacer of thegranular spacer is used as the spacer 70306 is described; however, theinvention is not limited thereto, and a columnar spacer may be used. Thecolumnar spacer may be provided for the first substrate 70301 or thesecond substrate 70307. Alternatively, a part of the spacers may beprovided for the first substrate 70301 and the other part thereof may beprovided for the second substrate 70307.

Note that a spacer may be mixed in the sealing material. Accordingly,the cell gap of the liquid crystal can be maintained constant moreeasily.

FIG. 74A is a cross-sectional view of a step of dropping the liquidcrystals 70309. Defoaming treatment is performed on the liquid crystals70309, and thereafter, the liquid crystals 70309 are dropped into theinside of the sealing material 70305.

FIG. 74B is a top plan view after the liquid crystals 70309 are dropped.Since the sealing material 70305 is formed along the periphery of thefirst substrate 70301, the liquid crystals 70309 are not leaked.

FIG. 74C is a cross-sectional view of a step of attaching the firstsubstrate 70301 and the second substrate 70307. The first substrate70301 and the second substrate 70307 are attached in a vacuum chamber.Then, the first substrate 70301 and the second substrate 70307 arepressurized so that a gap between the first substrate 70301 and thesecond substrate 70307 is constant. Thereafter, ultraviolet rayirradiation is performed on the sealing material 70305; thus, thesealing material 70305 is hardened. It is preferable to performultraviolet ray irradiation on the sealing material 70305 while adisplay portion is covered with a mask because deterioration of theliquid crystals 70309 due to ultraviolet rays can be prevented.Thereafter, realignment treatment is performed on the liquid crystals70309 by annealing treatment. Accordingly, the liquid crystal cell iscompleted.

Note that although this embodiment mode is described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, even more drawings can be formed by combining each part withanother part in the above-described drawings.

Similarly, the contents (or part of the contents) described in eachdrawing of this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, even more drawings can beformed by combining each part with part of another embodiment mode inthe drawings of this embodiment mode.

This embodiment mode shows an example of an embodied case of thecontents (or part of the contents) described in another embodiment mode,an example of slight transformation thereof, an example of partialmodification thereof, an example of improvement thereof, an example ofdetailed description thereof, an application example thereof, an exampleof related part thereof, or the like. Therefore, the contents describedin another embodiment mode can be freely applied to, combined with, orreplaced with this embodiment mode.

Embodiment Mode 15

In this embodiment mode, a structure and an operation of a pixel in adisplay device are described.

FIGS. 75A and 75B are timing charts showing an example of digital timegray scale driving. The timing chart of FIG. 75A shows a driving methodin the case where a signal writing period (an address period) to a pixeland a light-emitting period (a sustain period) are separated.

One frame period refers to a period for fully displaying an image forone display region. One frame period includes a plurality of subframeperiods, and one subframe period includes an address period and asustain period. Address periods T_(a1) to T_(a4) indicate time forwriting signals to pixels in all rows, and periods T_(b1) to T_(b4)indicate time for writing signals to pixels in one row (or one pixel).Sustain periods T_(s1) to T_(s4) indicate time for maintaining alighting state or a non-lighting state in accordance with a video signalwritten to the pixel, and a ratio of the length of the sustain periodsis set to satisfy T_(s1):T_(s2):T_(s3):T_(s4)=2³:2²:2¹:2⁰=8:4:2:1. Agray scale is expressed depending on in which sustain period lightemission is performed.

An operation is described. First, in the address period T_(a1), pixelselection signals are sequentially input to scan lines from a first row,and a pixel is selected. Then, while the pixel is selected, a videosignal is input to the pixel from a signal line. Then, when the videosignal is written to the pixel, the pixel maintains the signal until asignal is input again. Lighting and non-lighting of each pixel in thesustain period T_(s1) are controlled by the written video signal.Similarly, in each of the address periods T_(a2), T_(a3), and T_(a4), avideo signal is input to pixels, and lighting and non-lighting of eachpixel in each of the sustain periods T_(s2), T_(s3), and T_(s4) arecontrolled by the video signal. Then, in each subframe period, a pixelto which a signal for not lighting in the address period and forlighting when the sustain period starts after the address period ends iswritten is lit.

Here, the i-th pixel row is described with reference to FIG. 75B. First,in the address period T_(a1), pixel selection signals are input to thescan lines from a first row, and in a period T_(b1(i)) in the addressperiod T_(a1), a pixel in the i-th row is selected. Then, while thepixel in the i-th row is selected, a video signal is input to the pixelin the i-th row from a signal line. Then, when the video signal iswritten to the pixel in the i-th row, the pixel in the i-th rowmaintains the signal until a signal is input again. Lighting andnon-lighting of the pixel in the i-th row in the sustain period T_(s1)are controlled by the written video signal. Similarly, in each of theaddress periods T_(a2), T_(a3), and T_(a4), a video signal is input tothe pixel in the i-th row, and lighting and non-lighting of the pixel inthe i-th row in each of the sustain periods T_(s2), T_(s3), and T_(s4)are controlled by the video signal. Then, in each subframe period, apixel to which a signal for not lighting in the address period and forlighting when the sustain period starts after the address period ends iswritten is lit.

The case where 4-bit gray scales are expressed is described here;however, the number of bits and the number of gray scales are notlimited thereto. Note that lighting is not needed to be performed inorder of T_(s1), T_(s2), T_(s3), and T_(s4), and the order may be randomor light may be emitted by dividing any of the periods of T_(s1),T_(s2), T_(s3), and T_(s4) into a plurality of periods. A ratio oflighting times of T_(s1), T_(s2), T_(s3), and T_(s4) is not needed to bea power of two, and may be the same length or slightly different from apower of two.

Next, a driving method in the case where a period for writing a signalto a pixel (an address period) and a light-emitting period (a sustainperiod) are not separated is described. That is, a pixel in a row inwhich a writing operation of a video signal is completed maintains thesignal until another signal is written to the pixel (or the signal iserased). A period between the writing operation and writing of anothersignal to the pixel is referred to as data holding time. In the dataholding time, the pixel is lit or not lit in accordance with the videosignal written to the pixel. The same operations are performed until thelast row, and the address period ends. Then, a signal writing operationof the next subframe period starts sequentially from the row in whichthe data holding time ends.

As described above, in the case of the driving method in which a pixelis immediately lit or not lit in accordance with a video signal writtento the pixel when the signal writing operation is completed and the dataholding time starts, signals cannot be input to two rows at the sametime. Accordingly, address periods need to be prevented fromoverlapping, so that the data holding time cannot be made shorter. As aresult, it is difficult to perform high-level gray scale display.

Thus, the data holding time is set to be shorter than the address periodby provision of an erasing period. A driving method in the case wherethe data holding time which is shorter than the address period is set byprovision of an erasing period is described with reference to FIG. 76A.

First, in the address period T_(a1), pixel scan signals are input to thescan lines from a first row, and a pixel is selected. Then, while thepixel is selected, a video signal is input to the pixel from a signalline. Then, when the video signal is written to the pixel, the pixelmaintains the signal until a signal is input again. Lighting andnon-lighting of the pixel in the sustain period T_(s1) are controlled bythe written video signal. In the row in which a writing operation of avideo signal is completed, each pixel is immediately lit or not lit inaccordance with the written video signal. The same operations areperformed until the last row, and the address period T_(a1) ends. Then,a signal writing operation of the next subframe period startssequentially from the row in which the data holding time ends.Similarly, in each of the address periods T_(a2), T_(a3), and T_(a4), avideo signal is input to the pixel, and lighting and non-lighting of thepixel in each of the sustain periods T_(s2), T_(s3), and T_(s4) arecontrolled by the video signal. The end of the sustain period T_(s4) isset by the start of an erasing operation. This is because when a signalwritten to a pixel is erased in an erasing time T_(e) of each row, thepixel is forced to be not lit regardless of the video signal written tothe pixel in the address period until another signal is written to thepixel. That is, the data holding time ends from a pixel in which theerasing time T_(e) starts.

Here, the i-th pixel row is described with reference to FIG. 76B. In theaddress period T_(a1), pixel scan signals are input to the scan linesfrom a first row, and a pixel is selected. Then, in the periodT_(b1(i)), while the pixel in the i-th row is selected, a video signalis input to the pixel in the i-th row. Then, when the video signal iswritten to the pixel in the i-th row, the pixel in the i-th rowmaintains the signal until a signal is input again. Lighting andnon-lighting of the pixel in the i-th row in a sustain period T_(s1(i))are controlled by the written video signal. That is, the pixel in thei-th row is immediately lit or not lit in accordance with the videosignal written to the pixel after the writing operation of the videosignal to the i-th row is completed. Similarly, in each of the addressperiods T_(a2), T_(a3), and T_(a4), a video signal is input to the pixelin the i-th row, and lighting and non-lighting of the pixel in the i-throw in each of the sustain periods T_(s2), T_(s3), and T_(s4) arecontrolled by the video signal. The end of a sustain period T_(s4(i)) isset by the start of an erasing operation. This is because the pixel isforced to be not lit regardless of the video signal written to the pixelin the i-th row in an erasing time T_(e(i)) in the i-th row. That is,the data holding time of the pixel in the i-th row ends when the erasingtime T_(e(i)) starts.

Thus, a display device with a high-level gray scale and a high dutyratio (a ratio of a lighting period in one frame period) can beprovided, in which data holding time is shorter than an address periodwithout separating the address period and a sustain period. Sinceinstantaneous luminance can be lowered, reliability of a display elementcan be improved.

The case where 4-bit gray scales are expressed is described here;however, the number of bits and the number of gray scales are notlimited thereto. Note that lighting is not needed to be performed inorder of T_(s1), T_(s2), T_(s3), and T_(s4), and the order may be randomor light may be emitted by dividing any of the periods of T_(s1),T_(s2), T_(s3), and T_(s4) into a plurality of periods. A ratio oflighting times of T_(s1), T_(s2), T_(s3), and T_(s4) is not needed to bea power of two, and may be the same length or slightly different from apower of two.

Next, a structure and an operation of a pixel to which digital time grayscale driving can be applied are described.

FIG. 77 shows an example of a pixel structure to which digital time grayscale driving can be applied.

A pixel 80300 includes a switching transistor 80301, a drivingtransistor 80302, a light-emitting element 80304, and a capacitor 80303.A gate of the switching transistor 80301 is connected to a scan line80306, a first electrode (one of a source electrode and a drainelectrode) of the switching transistor 80301 is connected to a signalline 80305, and a second electrode (the other of the source electrodeand the drain electrode) of the switching transistor 80301 is connectedto a gate of the driving transistor 80302. The gate of the drivingtransistor 80302 is connected to a power supply line 80307 through thecapacitor 80303, a first electrode of the driving transistor 80302 isconnected to the power supply line 80307, and a second electrode of thedriving transistor 80302 is connected to a first electrode (a pixelelectrode) of the light-emitting element 80304. A second electrode ofthe light-emitting element 80304 corresponds to a common electrode80308.

Note that the second electrode (the common electrode 80308) of thelight-emitting element 80304 is set to have a low power supplypotential. The low power supply potential refers to a potentialsatisfying (the low power supply potential)<(a high power supplypotential) based on the high power supply potential set to the powersupply line 80307. As the low power supply potential, GND, 0 V, or thelike may be set, for example. In order to make the light-emittingelement 80304 emit light by applying a potential difference between thehigh power supply potential and the low power supply potential to thelight-emitting element 80304 so that current is supplied to thelight-emitting element 80304, each of the potentials is set so that thepotential difference between the high power supply potential and the lowpower supply potential is equal to or higher than the forward thresholdvoltage of the light-emitting element 80304.

Note that gate capacitance of the driving transistor 80302 may be usedas a substitute for the capacitor 80303, so that the capacitor 80303 canbe omitted. The gate capacitance of the driving transistor 80302 may beformed in a region where a source region, a drain region, an LDD region,or the like overlaps with the gate electrode. Alternatively, capacitancemay be formed between a channel formation region and the gate electrode.

When a pixel is selected by the scan line 80306, that is, when theswitching transistor 80301 is turned on, a video signal is input to thepixel from the signal line 80305. Then, a charge for voltagecorresponding to the video signal is stored in the capacitor 80303, andthe capacitor 80303 holds the voltage. The voltage is voltage betweenthe gate and the first electrode of the driving transistor 80302 andcorresponds to gate-source voltage V_(gs) of the driving transistor80302.

In general, an operation region of a transistor can be divided into alinear region and a saturation region. When the drain-source voltage isdenoted by V_(ds), the gate-source voltage is denoted by V_(gs), and thethreshold voltage is denoted by V_(th), a boundary between the linearregion and the saturation region is the time when (V_(gs)−V_(th))=V_(ds)is satisfied. In the case where (V_(gs)−V_(th))>V_(ds) is satisfied, thetransistor operates in the linear region, and a current value isdetermined in accordance with the levels of V_(ds) and V_(gs). On theother hand, in the case where (V_(gs)−V_(th))<V_(ds) is satisfied, thetransistor operates in the saturation region and ideally, a currentvalue hardly changes even when V_(ds) changes. That is, the currentvalue is determined only by the level of V_(gs).

Here, in the case of a voltage-input voltage driving method, a videosignal is input to the gate of the driving transistor 80302 so that thedriving transistor 80302 is in either of two states of beingsufficiently turned on and turned off. That is, the driving transistor80302 operates in the linear region.

Thus, when a video signal which makes the driving transistor 80302turned on is input, a power supply potential VDD set to the power supplyline 80307 is ideally set to the first electrode of the light-emittingelement 80304 without change.

That is, ideally, constant voltage is applied to the light-emittingelement 80304 to obtain constant luminance from the light-emittingelement 80304. Then, a plurality of subframe periods are provided in oneframe period. A video signal is written to a pixel in each subframeperiod, lighting and non-lighting of the pixel are controlled in eachsubframe period, and a gray scale is expressed by the sum of lightingsubframe periods.

Note that when the video signal by which the driving transistor 80302operates in the saturation region is input, current can be supplied tothe light-emitting element 80304. When the light-emitting element 80304is an element luminance of which is determined in accordance withcurrent, luminance decay due to deterioration of the light-emittingelement 80304 can be suppressed. Further, when the video signal is ananalog signal, current in accordance with the video signal can besupplied to the light-emitting element 80304. In this case, analog grayscale driving can be performed.

FIG. 78 shows another example of a pixel structure to which digital timegray scale driving can be applied.

A pixel 80400 includes a switching transistor 80401, a drivingtransistor 80402, a capacitor 80403, a light-emitting element 80404, anda rectifying element 80409. A gate of the switching transistor 80401 isconnected to a first scan line 80406, a first electrode (one of a sourceelectrode and a drain electrode) of the switching transistor 80401 isconnected to a signal line 80405, and a second electrode (the other ofthe source electrode and the drain electrode) of the switchingtransistor 80401 is connected to a gate of the driving transistor 80402.The gate of the driving transistor 80402 is connected to a power supplyline 80407 through the capacitor 80403, and is also connected to asecond scan line 80410 through the rectifying element 80409. A firstelectrode of the driving transistor 80402 is connected to the powersupply line 80407, and a second electrode of the driving transistor80402 is connected to a first electrode (a pixel electrode) of thelight-emitting element 80404. A second electrode of the light-emittingelement 80404 corresponds to a common electrode 80408.

The second electrode (the common electrode 80408) of the light-emittingelement 80404 is set to have a low power supply potential. Note that thelow power supply potential refers to a potential satisfying (the lowpower supply potential)<(a high power supply potential) based on thehigh power supply potential set to the power supply line 80407. As thelow power supply potential, GND, 0 V, or the like may be set, forexample. In order to make the light-emitting element 80404 emit light byapplying a potential difference between the high power supply potentialand the low power supply potential to the light-emitting element 80404so that current is supplied to the light-emitting element 80404, each ofthe potentials is set so that the potential difference between the highpower supply potential and the low power supply potential is equal to orhigher than the forward threshold voltage of the light-emitting element80404.

Note that gate capacitance of the driving transistor 80402 may be usedas a substitute for the capacitor 80403, so that the capacitor 80403 canbe omitted. The gate capacitance of the driving transistor 80402 may beformed in a region where a source region, a drain region, an LDD region,or the like overlaps with the gate electrode. Alternatively, capacitancemay be formed between a channel formation region and the gate electrode.

As the rectifying element 80409, a diode-connected transistor can beused. A PN junction diode, a PIN junction diode, a Schottky diode, adiode formed of a carbon nanotube, or the like may be used as well as adiode-connected transistor. A diode-connected transistor may be ann-channel transistor or a p-channel transistor.

The pixel 80400 is such that the rectifying element 80409 and the secondscan line 80410 are added to the pixel shown in FIG. 77. Accordingly,the switching transistor 80401, the driving transistor 80402, thecapacitor 80403, the light-emitting element 80404, the signal line80405, the first scan line 80406, the power supply line 80407, and thecommon electrode 80408 shown in FIG. 78 correspond to the switchingtransistor 80301, the driving transistor 80302, the capacitor 80303, thelight-emitting element 80304, the signal line 80305, the scan line80306, the power supply line 80307, and the common electrode 80308 shownin FIG. 77. Accordingly, a writing operation and a light-emittingoperation in FIG. 78 are similar to those described in FIG. 77, so thatdescription thereof is omitted.

An erasing operation is described. In the erasing operation, an H-levelsignal is input to the second scan line 80410. Thus, current is suppliedto the rectifying element 80409, and a gate potential of the drivingtransistor 80402 held by the capacitor 80403 can be set to a certainpotential. That is, the potential of the gate of the driving transistor80402 is set to a certain potential, and the driving transistor 80402can be forced to be turned off regardless of a video signal written tothe pixel.

Note that an L-level signal input to the second scan line 80410 has apotential such that current is not supplied to the rectifying element80409 when a video signal for non-lighting is written to the pixel. AnH-level signal input to the second scan line 80410 has a potential suchthat a potential to turn off the driving transistor 80402 can be set tothe gate regardless of a video signal written to the pixel.

FIG. 79 shows another example of a pixel structure to which digital timegray scale driving can be applied.

A pixel 80500 includes a switching transistor 80501, a drivingtransistor 80502, a capacitor 80503, a light-emitting element 80504, andan erasing transistor 80509. A gate of the switching transistor 80501 isconnected to a first scan line 80506, a first electrode (one of a sourceelectrode and a drain electrode) of the switching transistor 80501 isconnected to a signal line 80505, and a second electrode (the other ofthe source electrode and the drain electrode) of the switchingtransistor 80501 is connected to a gate of the driving transistor 80502.The gate of the driving transistor 80502 is connected to a power supplyline 80507 through the capacitor 80503, and is also connected to a firstelectrode of the erasing transistor 80509. A first electrode of thedriving transistor 80502 is connected to the power supply line 80507,and a second electrode of the driving transistor 80502 is connected to afirst electrode (a pixel electrode) of the light-emitting element 80504.A gate of the erasing transistor 80509 is connected to a second scanline 80510, and a second electrode of the erasing transistor 80509 isconnected to the power supply line 80507. A second electrode of thelight-emitting element 80504 corresponds to a common electrode 80508.

The second electrode (the common electrode 80508) of the light-emittingelement 80504 is set to have a low power supply potential. Note that thelow power supply potential refers to a potential satisfying (the lowpower supply potential)<(a high power supply potential) based on thehigh power supply potential set to the power supply line 80507. As thelow power supply potential, GND, 0 V, or the like may be set, forexample. In order to make the light-emitting element 80504 emit light byapplying a potential difference between the high power supply potentialand the low power supply potential to the light-emitting element 80504so that current is supplied to the light-emitting element 80504, each ofthe potentials is set so that the potential difference between the highpower supply potential and the low power supply potential is equal to orhigher than the forward threshold voltage of the light-emitting element80504.

Note that gate capacitance of the driving transistor 80502 may be usedas a substitute for the capacitor 80503, so that the capacitor 80503 canbe omitted. The gate capacitance of the driving transistor 80502 may beformed in a region where a source region, a drain region, an LDD region,or the like overlaps with the gate electrode. Alternatively, capacitancemay be formed between a channel formation region and the gate electrode.

The pixel 80500 is such that the erasing transistor 80509 and the secondscan line 80510 are added to the pixel shown in FIG. 77. Accordingly,the switching transistor 80501, the driving transistor 80502, thecapacitor 80503, the light-emitting element 80504, the signal line80505, the first scan line 80506, the power supply line 80507, and thecommon electrode 80508 shown in FIG. 79 correspond to the switchingtransistor 80301, the driving transistor 80302, the capacitor 80303, thelight-emitting element 80304, the signal line 80305, the scan line80306, the power supply line 80307, and the common electrode 80308 shownin FIG. 77. Accordingly, a writing operation and a light-emittingoperation in FIG. 79 are similar to those described in FIG. 77, so thatdescription thereof is omitted.

An erasing operation is described. In the erasing operation, an H-levelsignal is input to the second scan line 80510. Thus, the erasingtransistor 80509 is turned on, so that the gate and the first electrodeof the driving transistor 80502 can have the same potential. That is,V_(gs) of the driving transistor 80502 can be 0 V. Accordingly, thedriving transistor 80502 can be forced to be turned off.

Next, a structure and an operation of a pixel called a threshold voltagecompensation pixel are described. The threshold voltage compensationpixel can be applied to digital time gray scale driving and analog grayscale driving.

FIG. 80 shows an example of a structure of a pixel called a thresholdvoltage compensation pixel.

The pixel shown in FIG. 80 includes a driving transistor 80600, a firstswitch 80601, a second switch 80602, a third switch 80603, a firstcapacitor 80604, a second capacitor 80605, and a light-emitting element80620. A gate of the driving transistor 80600 is connected to a signalline 80611 through the first capacitor 80604 and the first switch 80601in this order. Further, the gate of the driving transistor 80600 isconnected to a power supply line 80612 through the second capacitor80605. A first electrode of the driving transistor 80600 is connected tothe power supply line 80612. A second electrode of the drivingtransistor 80600 is connected to a first electrode of the light-emittingelement 80620 through the third switch 80603. Further, the secondelectrode of the driving transistor 80600 is connected to the gate ofthe driving transistor 80600 through the second switch 80602. A secondelectrode of the light-emitting element 80620 corresponds to a commonelectrode 80621.

The second electrode of the light-emitting element 80620 is set to a lowpower supply potential. Note that the low power supply potential refersto a potential satisfying (the low power supply potential)<(a high powersupply potential) based on the high power supply potential set to thepower supply line 80612. As the low power supply potential, GND, 0 V, orthe like may be set, for example. In order to make the light-emittingelement 80620 emit light by applying a potential difference between thehigh power supply potential and the low power supply potential to thelight-emitting element 80620 so that current is supplied to thelight-emitting element 80620, each of the potentials is set so that thepotential difference between the high power supply potential and the lowpower supply potential is equal to or higher than the forward thresholdvoltage of the light-emitting element 80620. Note that gate capacitanceof the driving transistor 80600 may be used as a substitute for thesecond capacitor 80605, so that the second capacitor 80605 can beomitted. The gate capacitance of the driving transistor 80600 may beformed in a region where a source region, a drain region, an LDD region,or the like overlaps with the gate electrode. Alternatively, capacitancemay be formed between a channel formation region and the gate electrode.Note that on/off of the first switch 80601, the second switch 80602, andthe third switch 80603 is controlled by a first scan line 80613, asecond scan line 80615, and a third scan line 80614, respectively.

A method for driving the pixel shown in FIG. 80 is described in which anoperation period is divided into an initialization period, a datawriting period, a threshold detection period, and a light-emittingperiod.

In the initialization period, the second switch 80602 and the thirdswitch 80603 are turned on. Then, a potential of the gate of the drivingtransistor 80600 becomes lower than at least a potential of the powersupply line 80612. At this time, the first switch 80601 may be in an onstate or an off state. Note that the initialization period is notnecessarily required.

In the threshold detection period, a pixel is selected by the first scanline 80613. That is, the first switch 80601 is turned on, and constantvoltage is input from the signal line 80611. At this time, the secondswitch 80602 is turned on and the third switch 80603 is turned off.Accordingly, the driving transistor 80600 is diode-connected, and thesecond electrode and the gate of the driving transistor 80600 are placedin a floating state. Then, a potential of the gate of the drivingtransistor 80600 becomes a value obtained by subtracting the thresholdvoltage of the driving transistor 80600 from the potential of the powersupply line 80612. Thus, the threshold voltage of the driving transistor80600 is held in the first capacitor 80604. A potential differencebetween the potential of the gate of the driving transistor 80600 andthe constant voltage input from the signal line 80611 is held in thesecond capacitor 80605.

In the data writing period, a video signal (voltage) is input from thesignal line 80611. At this time, the first switch 80601 is kept on, thesecond switch 80602 is turned off, and the third switch 80603 is keptoff. Since the gate of the driving transistor 80600 is in a floatingstate, the potential of the gate of the driving transistor 80600 changesdepending on a potential difference between the constant voltage inputfrom the signal line 80611 in the threshold detection period and thevideo signal input from the signal line 80611 in the data writingperiod. For example, when (a capacitance value of the first capacitor80604)<<(a capacitance value of the second capacitor 80605) issatisfied, the potential of the gate of the driving transistor 80600 inthe data writing period is approximately equal to the sum of a potentialdifference (the amount of change) between the potential of the signalline 80611 in the threshold detection period and the potential of thesignal line 80611 in the data writing period, and a value obtained bysubtracting the threshold voltage of the driving transistor 80600 fromthe potential of the power supply line 80612. That is, the potential ofthe gate of the driving transistor 80600 becomes a potential obtained bycompensating the threshold voltage of the driving transistor 80600.

In the light-emitting period, current in accordance with a potentialdifference (V_(gs)) between the gate of the driving transistor 80600 andthe power supply line 80612 is supplied to the light-emitting element80620. At this time, the first switch 80601 is turned off, the secondswitch 80602 is kept off, and the third switch 80603 is turned on. Notethat current flowing to the light-emitting element 80620 is constantregardless of the threshold voltage of the driving transistor 80600.

Note that a pixel structure of the present invention is not limited tothat shown in FIG. 80. For example, a switch, a resistor, a capacitor, atransistor, a logic circuit, or the like may be added to the pixel shownin FIG. 80. For example, the second switch 80602 may include a p-channeltransistor or an n-channel transistor, the third switch 80603 mayinclude a transistor with polarity different from that of the secondswitch 80602, and the second switch 80602 and the third switch 80603 maybe controlled by the same scan line.

A structure and an operation of a pixel called a current input pixel aredescribed. The current input pixel can be applied to digital gray scaledriving and analog gray scale driving.

FIG. 81 shows an example of a structure of a pixel called a currentinput pixel.

The pixel shown in FIG. 81 includes a driving transistor 80700, a firstswitch 80701, a second switch 80702, a third switch 80703, a capacitor80704, and a light-emitting element 80730. A gate of the drivingtransistor 80700 is connected to a signal line 80711 through the secondswitch 80702 and the first switch 80701 in this order. Further, the gateof the driving transistor 80700 is connected to a power supply line80712 through the capacitor 80704. A first electrode of the drivingtransistor 80700 is connected to the power supply line 80712. A secondelectrode of the driving transistor 80700 is connected to the signalline 80711 through the first switch 80701. Further, the second electrodeof the driving transistor 80700 is connected to a first electrode of thelight-emitting element 80730 through the third switch 80703. A secondelectrode of the light-emitting element 80730 corresponds to a commonelectrode 80731.

The second electrode of the light-emitting element 80730 is set to a lowpower supply potential. Note that the low power supply potential refersto a potential satisfying (the low power supply potential)<(a high powersupply potential) based on the high power supply potential set to thepower supply line 80712. As the low power supply potential, GND, 0 V, orthe like may be set, for example. In order to make the light-emittingelement 80730 emit light by applying a potential difference between thehigh power supply potential and the low power supply potential to thelight-emitting element 80730 so that current is supplied to thelight-emitting element 80730, each of the potentials is set so that thepotential difference between the high power supply potential and the lowpower supply potential is equal to or higher than the forward thresholdvoltage of the light-emitting element 80730. Note that gate capacitanceof the driving transistor 80700 may be used as a substitute for thecapacitor 80704, so that the capacitor 80704 can be omitted. The gatecapacitance of the driving transistor 80700 may be formed in a regionwhere a source region, a drain region, an LDD region, or the likeoverlaps with the gate electrode. Alternatively, capacitance may beformed between a channel formation region and the gate electrode. Notethat on/off of the first switch 80701, the second switch 80702, and thethird switch 80703 is controlled by a first scan line 80713, a secondscan line 80714, and a third scan line 80715, respectively.

A method for driving the pixel shown in FIG. 81 is described in which anoperation period is divided into a data writing period and alight-emitting period.

In the data writing period, the pixel is selected by the first scan line80713. That is, the first switch 80701 is turned on, and current isinput as a video signal from the signal line 80711. At this time, thesecond switch 80702 is turned on and the third switch 80703 is turnedoff. Accordingly, a potential of the gate of the driving transistor80700 becomes a potential in accordance with the video signal. That is,voltage between the gate electrode and the source electrode of thedriving transistor 80700, by which the driving transistor 80700 suppliesthe same current as the video signal, is held in the capacitor 80704.

Next, in the light-emitting period, the first switch 80701 and thesecond switch 80702 are turned off, and the third switch 80703 is turnedon. Thus, current with the same value as the video signal is supplied tothe light-emitting element 80730.

Note that the present invention is not limited to the pixel structureshown in FIG. 81. For example, a switch, a resistor, a capacitor, atransistor, a logic circuit, or the like may be added to the pixel shownin FIG. 81. For example, the first switch 80701 may include a p-channeltransistor or an n-channel transistor, the second switch 80702 mayinclude a transistor with the same polarity as the first switch 80701,and the first switch 80701 and the second switch 80702 may be controlledby the same scan line. The second switch 80702 may be provided betweenthe gate of the driving transistor 80700 and the signal line 80711.

Note that although this embodiment mode is described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in each drawing in this embodiment modewith part of another embodiment mode.

Note that this embodiment mode shows examples of embodying, slightlytransforming, partially modifying, improving, describing in detail, orapplying the contents (or part of the contents) described in anotherembodiment mode, an example of related part thereof, or the like.Therefore, the contents described in another embodiment mode can befreely applied to, combined with, or replaced with this embodiment mode.

Embodiment Mode 16

In this embodiment mode, a pixel structure of a display device isdescribed. In particular, a pixel structure of a display device using anorganic EL element is described.

FIG. 82A shows an example of a top plan view (a layout diagram) of apixel including two transistors. FIG. 82B shows an example of across-sectional view along X-X′ in FIG. 82A.

FIG. 82A shows a first transistor 60105, a first wiring 60106, a secondwiring 60107, a second transistor 60108, a third wiring 60111, a counterelectrode 60112, a capacitor 60113, a pixel electrode 60115, a partitionwall 60116, an organic conductive film 60117, an organic thin film60118, and a substrate 60119. Note that it is preferable that the firsttransistor 60105 be used as a switching transistor, the first wiring60106 as a gate signal line, the second wiring 60107 as a source signalline, the second transistor 60108 as a driving transistor, and the thirdwiring 60111 as a current supply line.

A gate electrode of the first transistor 60105 is electrically connectedto the first wiring 60106. One of a source electrode and a drainelectrode of the first transistor 60105 is electrically connected to thesecond wiring 60107. The other of the source electrode and the drainelectrode of the first transistor 60105 is electrically connected to agate electrode of the second transistor 60108 and one electrode of thecapacitor 60113. Note that the gate electrode of the first transistor60105 includes a plurality of gate electrodes. Accordingly, leakagecurrent in the off state of the first transistor 60105 can be reduced.

One of a source electrode and a drain electrode of the second transistor60108 is electrically connected to the third wiring 60111, and the otherof the source electrode and the drain electrode of the second transistor60108 is electrically connected to the pixel electrode 60115.Accordingly, current flowing to the pixel electrode 60115 can becontrolled by the second transistor 60108.

The organic conductive film 60117 is provided over the pixel electrode60115, and the organic thin film 60118 (an organic compound layer) isprovided thereover. The counter electrode 60112 is provided over theorganic thin film 60118 (the organic compound layer). Note that thecounter electrode 60112 may be formed over the entire surface to beconnected to all the pixels in common, or may be patterned using ashadow mask or the like.

Light emitted from the organic thin film 60118 (the organic compoundlayer) is transmitted through either the pixel electrode 60115 or thecounter electrode 60112.

In FIG. 82B, the case where light is emitted to the pixel electrodeside, that is, a side on which the transistor and the like are formed isreferred to as bottom emission; and the case where light is emitted tothe counter electrode side is referred to as top emission.

In the case of bottom emission, it is preferable that the pixelelectrode 60115 be formed of a light-transmitting conductive film. Onthe other hand, in the case of top emission, it is preferable that thecounter electrode 60112 be formed of a light-transmitting conductivefilm.

In a light-emitting device for color display, EL elements havingrespective light emission colors of RGB may be separately formed, or anEL element with a single color may be formed over an entire surface andlight emission of RGB can be obtained by using a color filter.

Note that the structures shown in FIGS. 82A and 82B are examples, andvarious structures can be employed for a pixel layout, a cross-sectionalstructure, a stacking order of electrodes of an EL element, and thelike, other than the structures shown in FIGS. 82A and 82B. Further, asa light-emitting element, various elements such as a crystalline elementsuch as an LED, and an element formed of an inorganic thin film can beused as well as the element formed of the organic thin film shown in thedrawing.

FIG. 83A shows an example of a top plan view (a layout diagram) of apixel including three transistors. FIG. 83B shows an example of across-sectional view along X-X′ in FIG. 83A.

FIG. 83A shows a substrate 60200, a first wiring 60201, a second wiring60202, a third wiring 60203, a fourth wiring 60204, a first transistor60205, a second transistor 60206, a third transistor 60207, a pixelelectrode 60208, a partition wall 60211, an organic conductive film60212, an organic thin film 60213, and a counter electrode 60214. Notethat it is preferable that the first wiring 60201 be used as a sourcesignal line, the second wiring 60202 as a gate signal line for writing,the third wiring 60203 as a gate signal line for erasing, the fourthwiring 60204 as a current supply line, the first transistor 60205 as aswitching transistor, the second transistor 60206 as an erasingtransistor, and the third transistor 60207 as a driving transistor.

A gate electrode of the first transistor 60205 is electrically connectedto the second wiring 60202. One of a source electrode and a drainelectrode of the first transistor 60205 is electrically connected to thefirst wiring 60201. The other of the source electrode and the drainelectrode of the first transistor 60205 is electrically connected to agate electrode of the third transistor 60207. Note that the gateelectrode of the first transistor 60205 includes a plurality of gateelectrodes. Accordingly, leakage current in the off state of the firsttransistor 60205 can be reduced.

A gate electrode of the second transistor 60206 is electricallyconnected to the third wiring 60203. One of a source electrode and adrain electrode of the second transistor 60206 is electrically connectedto the fourth wiring 60204. The other of the source electrode and thedrain electrode of the second transistor 60206 is electrically connectedto the gate electrode of the third transistor 60207. Note that the gateelectrode of the second transistor 60206 includes a plurality of gateelectrodes. Accordingly, leakage current in the off state of the secondtransistor 60206 can be reduced.

One of a source electrode and a drain electrode of the third transistor60207 is electrically connected to the fourth wiring 60204, and theother of the source electrode and the drain electrode of the thirdtransistor 60207 is electrically connected to the pixel electrode 60208.Accordingly, current flowing to the pixel electrode 60208 can becontrolled by the third transistor 60207.

The organic conductive film 60212 is provided over the pixel electrode60208, and the organic thin film 60213 (an organic compound layer) isprovided thereover. The counter electrode 60214 is provided over theorganic thin film 60213 (the organic compound layer). Note that thecounter electrode 60214 may be formed over the entire surface to beconnected to all the pixels in common, or may be patterned using ashadow mask or the like.

Light emitted from the organic thin film 60213 (the organic compoundlayer) is transmitted through either the pixel electrode 60208 or thecounter electrode 60214.

In FIG. 83B, the case where light is emitted to the pixel electrodeside, that is, a side on which the transistor and the like are formed isreferred to as bottom emission; and the case where light is emitted tothe counter electrode side is referred to as top emission.

In the case of bottom emission, it is preferable that the pixelelectrode 60208 be formed of a light-transmitting conductive film. Onthe other hand, in the case of top emission, it is preferable that thecounter electrode 60214 be formed of a light-transmitting conductivefilm.

In a light-emitting device for color display, EL elements havingrespective light emission colors of RGB may be separately formed, or anEL element with a single color may be formed over an entire surface andlight emission of RGB can be obtained by using a color filter.

Note that the structures shown in FIGS. 83A and 83B are examples, andvarious structures can be employed for a pixel layout, a cross-sectionalstructure, a stacking order of electrodes of an EL element, and thelike, other than the structures shown in FIGS. 83A and 83B. Further, asa light-emitting element, various elements such as a crystalline elementsuch as an LED, and an element formed of an inorganic thin film can beused as well as the element formed of the organic thin film shown in thedrawing.

FIG. 84A shows an example of a top plan view (a layout diagram) of apixel including four transistors. FIG. 84B shows an example of across-sectional view along X-X′ in FIG. 84A.

FIG. 84A shows a substrate 60300, a first wiring 60301, a second wiring60302, a third wiring 60303, a fourth wiring 60304, a first transistor60305, a second transistor 60306, a third transistor 60307, a fourthtransistor 60308, a pixel electrode 60309, a fifth wiring 60311, a sixthwiring 60312, a partition wall 60321, an organic conductive film 60322,an organic thin film 60323, and a counter electrode 60324. Note that itis preferable that the first wiring 60301 be used as a source signalline, the second wiring 60302 as a gate signal line for writing, thethird wiring 60303 as a gate signal line for erasing, the fourth wiring60304 as a signal line for reverse bias, the first transistor 60305 as aswitching transistor, the second transistor 60306 as an erasingtransistor, the third transistor 60307 as a driving transistor, thefourth transistor 60308 as a transistor for reverse bias, the fifthwiring 60311 as a current supply line, and the sixth wiring 60312 as apower supply line for reverse bias.

A gate electrode of the first transistor 60305 is electrically connectedto the second wiring 60302. One of a source electrode and a drainelectrode of the first transistor 60305 is electrically connected to thefirst wiring 60301. The other of the source electrode and the drainelectrode of the first transistor 60305 is electrically connected to agate electrode of the third transistor 60307. Note that the gateelectrode of the first transistor 60305 includes a plurality of gateelectrodes. Accordingly, leakage current in the off state of the firsttransistor 60305 can be reduced.

A gate electrode of the second transistor 60306 is electricallyconnected to the third wiring 60303. One of a source electrode and adrain electrode of the second transistor 60306 is electrically connectedto the fifth wiring 60311. The other of the source electrode and thedrain electrode of the second transistor 60306 is electrically connectedto the gate electrode of the third transistor 60307. Note that the gateelectrode of the second transistor 60306 includes a plurality of gateelectrodes. Accordingly, leakage current in the off state of the secondtransistor 60306 can be reduced.

One of a source electrode and a drain electrode of the third transistor60307 is electrically connected to the fifth wiring 60311, and the otherof the source electrode and the drain electrode of the third transistor60307 is electrically connected to the pixel electrode 60309.Accordingly, current flowing to the pixel electrode 60309 can becontrolled by the third transistor 60307.

A gate electrode of the fourth transistor 60308 is electricallyconnected to the fourth wiring 60304. One of a source electrode and adrain electrode of the fourth transistor 60308 is electrically connectedto the sixth wiring 60312. The other of the source electrode and thedrain electrode of the fourth transistor 60308 is electrically connectedto the pixel electrode 60309. Accordingly, a potential of the pixelelectrode 60309 can be controlled by the fourth transistor 60308, sothat reverse bias can be applied to the organic conductive film 60322and the organic thin film 60323. When reverse bias is applied to alight-emitting element including the organic conductive film 60322, theorganic thin film 60323, and the like, reliability of the light-emittingelement can be significantly improved.

The organic conductive film 60322 is provided over the pixel electrode60309, and the organic thin film 60323 (an organic compound layer) isprovided thereover. The counter electrode 60324 is provided over theorganic thin film 60323 (the organic compound layer). Note that thecounter electrode 60324 may be formed over the entire surface to beconnected to all the pixels in common, or may be patterned using ashadow mask or the like.

Light emitted from the organic thin film 60323 (the organic compoundlayer) is transmitted through either the pixel electrode 60309 or thecounter electrode 60324.

In FIG. 84B, the case where light is emitted to the pixel electrodeside, that is, a side on which the transistor and the like are formed isreferred to as bottom emission; and the case where light is emitted tothe counter electrode side is referred to as top emission.

In the case of bottom emission, it is preferable that the pixelelectrode 60309 be formed of a light-transmitting conductive film. Onthe other hand, in the case of top emission, it is preferable that thecounter electrode 60324 be formed of a light-transmitting conductivefilm.

In a light-emitting device for color display, EL elements havingrespective light emission colors of RGB may be separately formed, or anEL element with a single color may be formed over an entire surface andlight emission of RGB can be obtained by using a color filter.

Note that the structures shown in FIGS. 84A and 84B are examples, andvarious structures can be employed for a pixel layout, a cross-sectionalstructure, a stacking order of electrodes of an EL element, and thelike, other than the structures shown in FIGS. 84A and 84B. Further, asa light-emitting element, various elements such as a crystalline elementsuch as an LED, and an element formed of an inorganic thin film can beused as well as the element formed of the organic thin film shown in thedrawing.

Note that although this embodiment mode is described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in each drawing in this embodiment modewith part of another embodiment mode.

Note that this embodiment mode shows examples of embodying, slightlytransforming, partially modifying, improving, describing in detail, orapplying the contents (or part of the contents) described in anotherembodiment mode, an example of related part thereof, or the like.Therefore, the contents described in another embodiment mode can befreely applied to, combined with, or replaced with this embodiment mode.

Embodiment Mode 17

In this embodiment mode, a structure of an EL element is described. Inparticular, a structure of an organic EL element is described.

A structure of a mixed junction EL element is described. As an example,a structure is described, which includes a layer (a mixed layer) inwhich a plurality of materials among a hole injecting material, a holetransporting material, a light-emitting material, an electrontransporting material, an electron injecting material, and the like aremixed (hereinafter referred to as a mixed junction type EL element),which is different from a stacked-layer structure where a hole injectinglayer formed of a hole injecting material, a hole transporting layerformed of a hole transporting material, a light-emitting layer formed ofa light-emitting material, an electron transporting layer formed of anelectron transporting material, an electron injecting layer formed of anelectron injecting material, and the like are clearly distinguished.

FIGS. 85A to 85E are schematic views each showing a structure of a mixedjunction type EL element. Note that a layer interposed between an anode190101 and a cathode 190102 corresponds to an EL layer.

FIG. 85A shows a structure in which an EL layer includes a holetransporting region 190103 formed of a hole transporting material and anelectron transporting region 190104 formed of an electron transportingmaterial. The hole transporting region 190103 is closer to the anodethan the electron transporting region 190104. A mixed region 190105including both the hole transporting material and the electrontransporting material is provided between the hole transporting region190103 and the electron transporting region 190104.

In a direction from the anode 190101 to the cathode 190102, aconcentration of the hole transporting material in the mixed region190105 is decreased and a concentration of the electron transportingmaterial in the mixed region 190105 is increased.

Note that a concentration gradient can be freely set. For example, aratio of concentrations of each functional material may be changed (aconcentration gradient may be formed) in the mixed region 190105including both the hole transporting material and the electrontransporting material, without including the hole transporting region190103 formed of only the hole transporting material. Alternatively, aratio of concentrations of each functional material may be changed (aconcentration gradient may be formed) in the mixed region 190105including both the hole transporting material and the electrontransporting material, without including the hole transporting region190103 formed of only the hole transporting material and the electrontransporting region 190104 formed of only the electron transportingmaterial. Further alternatively, a ratio of concentrations may bechanged depending on a distance from the anode or the cathode. Note thatthe ratio of concentrations may be changed continuously.

A region 190106 to which a light-emitting material is added is includedin the mixed region 190105. A light emission color of the EL element canbe controlled by the light-emitting material. Further, carriers can betrapped by the light-emitting material. As the light-emitting material,various fluorescent dyes as well as a metal complex having a quinolineskeleton, a benzoxazole skeleton, or a benzothiazole skeleton can beused. The light emission color of the EL element can be controlled byadding the light-emitting material.

As the anode 190101, an electrode material having a high work functionis preferably used in order to inject holes efficiently. For example, atransparent electrode formed of indium tin oxide (ITO), indium zincoxide (IZO), ZnO, SnO₂, In₂O₃, or the like can be used. When alight-transmitting property is not needed, the anode 190101 may beformed of an opaque metal material.

As the hole transporting material, an aromatic amine compound or thelike can be used.

As the electron transporting material, a metal complex having aquinoline derivative, 8-quinolinol, or a derivative thereof as a ligand(especially tris(8-quinolinolato)aluminum (Alq₃)), or the like can beused.

As the cathode 190102, an electrode material having a low work functionis preferably used in order to inject electrons efficiently. A metalsuch as aluminum, indium, magnesium, silver, calcium, barium, or lithiumcan be used by itself. Alternatively, an alloy of the aforementionedmetal or an alloy of the aforementioned metal and another metal may beused.

FIG. 85B is the schematic view of the structure of the EL element, whichis different from that of FIG. 85A. Note that the same portions as thosein FIG. 85A are denoted by the same reference numerals, and descriptionthereof is omitted.

In FIG. 85B, a region to which a light-emitting material is added is notincluded. However, when a material (electron-transporting andlight-emitting material) having both an electron transporting propertyand a light-emitting property, for example,tris(8-quinolinolato)aluminum (Alq3) is used as a material added to theelectron transporting region 190104, light emission can be performed.

Alternatively, as a material added to the hole transporting region190103, a material (a hole-transporting and light-emitting material)having both a hole transporting property and a light-emitting propertymay be used.

FIG. 85C is the schematic view of the structure of the EL element, whichis different from those of FIGS. 85A and 85B. Note that the sameportions as those in FIGS. 85A and 85B are denoted by the same referencenumerals, and description thereof is omitted.

In FIG. 85C, a region 190107 included in the mixed region 190105 isprovided, to which a hole blocking material having a larger energydifference between the highest occupied molecular orbital and the lowestunoccupied molecular orbital than the hole transporting material isadded. The region 190107 to which the hole blocking material is added isprovided closer to the cathode 190102 than the region 190106 in themixed region 190105, to which the light-emitting material is added;thus, a recombination rate of carriers can be increased, and lightemission efficiency can be increased. The structure provided with theregion 190107 to which the hole blocking material is added is especiallyeffective in an EL element which utilizes light emission(phosphorescence) by a triplet exciton.

FIG. 85D is the schematic view of the structure of the EL element, whichis different from those of FIGS. 85A to 85C. Note that the same portionsas those in FIGS. 85A to 85C are denoted by the same reference numerals,and description thereof is omitted.

In FIG. 85D, a region 190108 included in the mixed region 190105 isprovided, to which an electron blocking material having a larger energydifference between the highest occupied molecular orbital and the lowestunoccupied molecular orbital than the electron transporting material isadded. The region 190108 to which the electron blocking material isadded is provided closer to the anode 190101 than the region 190106 inthe mixed region 190105, to which the light-emitting material is added;thus, a recombination rate of carriers can be increased, and lightemission efficiency can be increased. The structure provided with theregion 190108 to which the electron blocking material is added isespecially effective in an EL element which utilizes light emission(phosphorescence) by a triplet exciton.

FIG. 85E is the schematic view of the structure of the mixed junctiontype EL element, which is different from those of FIGS. 85A to 85D. FIG.85E shows an example of a structure where a region 190109 to which ametal material is added is included in part of an EL layer in contactwith an electrode of the EL element. In FIG. 85E, the same portions asthose in FIGS. 85A to 85D are denoted by the same reference numerals,and description thereof is omitted. In the structure shown in FIG. 85E,MgAg (an Mg—Ag alloy) may be used as the cathode 190102, and the region190109 to which an Al (aluminum) alloy is added may be included in aregion of the electron transporting region 190104 to which the electrontransporting material is added, which is in contact with the cathode190102, for example. With the aforementioned structure, oxidation of thecathode can be prevented, and electron injection efficiency from thecathode can be increased. Accordingly, the lifetime of the mixedjunction type EL element can be extended. Further, driving voltage canbe lowered.

As a method of forming the mixed junction type EL element, aco-evaporation method or the like can be used.

In the mixed junction type EL elements as shown in FIGS. 85A to 85E, aclear interface between the layers does not exist, and chargeaccumulation can be reduced. Accordingly, the lifetime of the EL elementcan be extended. Further, driving voltage can be lowered.

Note that the structures shown in FIGS. 85A to 85E can be implemented infree combination with each other.

In addition, a structure of the mixed junction type EL element is notlimited to those described above. A known structure can be freely used.

An organic material which forms an EL layer of an EL element may be alow molecular material or a high molecular material. Alternatively, bothof the materials may be used. When a low molecular material is used foran organic compound material, a film can be formed by an evaporationmethod. When a high molecular material is used for the EL layer, thehigh molecular material is dissolved in a solvent and a film can beformed by a spin coating method or an inkjet method.

The EL layer may be formed of a middle molecular material. In thisspecification, a middle molecule organic light-emitting material refersto an organic light-emitting material without a sublimation property andwith a polymerization degree of approximately 20 or less. When a middlemolecular material is used for the EL layer, a film can be formed by aninkjet method or the like.

Note that a low molecular material, a high molecular material, and amiddle molecular material may be used in combination.

An EL element may utilize either light emission (fluorescence) by asinglet exciton or light emission (phosphorescence) by a tripletexciton.

Next, an evaporation device for manufacturing a display device isdescribed with reference to drawings.

A display device may be manufactured to include an EL layer. The ELlayer is formed including at least partially a material which exhibitselectroluminescence. The EL layer may be formed of a plurality of layershaving different functions. In this case, the EL layer may be formed ofa combination of layers having different functions, which are alsoreferred to as a hole injecting and transporting layer, a light-emittinglayer, an electron injecting and transporting layer, and the like.

FIG. 86 shows a structure of an evaporation device for forming an ELlayer over an element substrate provided with a transistor. In theevaporation device, a plurality of treatment chambers are connected totransfer chambers 190260 and 190261. Each treatment chamber includes aloading chamber 190262 for supplying a substrate, an unloading chamber190263 for collecting the substrate, a heat treatment chamber 190268, aplasma treatment chamber 190272, deposition treatment chambers 190269 to190271 and 190273 to 190275 for depositing an EL material, and adeposition treatment chamber 190276 for forming a conductive film whichis formed of aluminum or contains aluminum as its main component as oneelectrode of an EL element. Gate valves 190277 a to 1902771 are providedbetween the transfer chambers and the treatment chambers, so that thepressure in each treatment chamber can be controlled independently, andcross contamination between the treatment chambers is prevented.

A substrate introduced into the transfer chamber 190260 from the loadingchamber 190262 is transferred to a predetermined treatment chamber by anarm type transfer means 190266 capable of rotating. The substrate istransferred from a certain treatment chamber to another treatmentchamber by the transfer means 190266. The transfer chambers 190260 and190261 are connected by the deposition treatment chamber 190270 at whichthe substrate is transported by the transfer means 190266 and a transfermeans 190267.

Each treatment chamber connected to the transfer chambers 190260 and190261 is maintained in a reduced pressure state. Accordingly, in theevaporation device, deposition treatment of an EL layer is continuouslyperformed without exposing the substrate to the room air. A displaypanel in which formation of the EL layer is finished is deteriorated dueto moisture or the like in some cases. Accordingly, in the evaporationdevice, a sealing treatment chamber 190265 for performing sealingtreatment before exposure to the room air in order to maintain thequality is connected to the transfer chamber 190261. Since the sealingtreatment chamber 190265 is under atmospheric pressure or reducedpressure near atmospheric pressure, an intermediate treatment chamber190264 is also provided between the transfer chamber 190261 and thesealing treatment chamber 190265. The intermediate treatment chamber190264 is provided for transporting the substrate and buffering thepressure between the chambers.

An exhaust means is provided in the loading chamber, the unloadingchamber, the transfer chamber, and the deposition treatment chamber inorder to maintain reduced pressure in the chamber. As the exhaust means,various vacuum pumps such as a dry pump, a turbo-molecular pump, and adiffusion pump can be used.

In the evaporation device of FIG. 86, the number of treatment chambersconnected to the transfer chambers 190260 and 190261 and structuresthereof can be combined as appropriate in accordance with astacked-layer structure of the EL element. An example of a combinationis described below.

In the heat treatment chamber 190268, degasification treatment isperformed by heating a substrate over which a lower electrode, aninsulating partition wall, or the like is formed. In the plasmatreatment chamber 190272, a surface of the lower electrode is treatedwith a rare gas or oxygen plasma. This plasma treatment is performed forcleaning the surface, stabilizing a surface state, or stabilizing aphysical or chemical state (e.g., a work function) of the surface.

The deposition treatment chamber 190269 is for forming an electrodebuffer layer which is in contact with one electrode of the EL element.The electrode buffer layer has a carrier injection property (holeinjection or electron injection) and suppresses generation of ashort-circuit or a black spot defect of the EL element. Typically, theelectrode buffer layer is formed of an organic-inorganic hybridmaterial, has a resistivity of 5×10⁴ to 1×10⁶ Ωcm, and is formed havinga thickness of 30 to 300 nm. Note that the deposition treatment chamber190271 is for forming a hole transporting layer.

A light-emitting layer in an EL element has a different structurebetween the case of emitting single color light and the case of emittingwhite light. Deposition treatment chambers in the evaporation device arepreferably arranged depending on the structure. For example, when threekinds of EL elements each having a different light emission color areformed in a display panel, it is necessary to form light-emitting layerscorresponding to respective light emission colors. In this case, thedeposition treatment chamber 190270 can be used for forming a firstlight-emitting layer, the deposition treatment chamber 190273 can beused for forming a second light-emitting layer, and the depositiontreatment chamber 190274 can be used for forming a third light-emittinglayer. By using different deposition treatment chambers for respectivelight-emitting layers, cross contamination due to differentlight-emitting materials can be prevented, and throughput of thedeposition treatment can be improved.

Note that three kinds of EL elements each having a different lightemission color may be sequentially deposited in each of the depositiontreatment chambers 190270, 190273, and 190274. In this case, evaporationis performed by moving a shadow mask depending on a region to bedeposited.

When an EL element which emits white light is formed, the EL element isformed by vertically stacking light-emitting layers of different lightemission colors. In this case also, the element substrate can besequentially transferred through the deposition treatment chambers sothat each light-emitting layer is formed. Alternatively, differentlight-emitting layers can be formed continuously in the same depositiontreatment chamber.

In the deposition treatment chamber 190276, an electrode is formed overthe EL layer. The electrode can be formed by an electron beamevaporation method or a sputtering method, and preferably by aresistance heating evaporation method.

The element substrate in which formation of the electrode is finished istransferred to the sealing treatment chamber 190265 through theintermediate treatment chamber 190264. The sealing treatment chamber190265 is filled with an inert gas such as helium, argon, neon, ornitrogen, and a sealing substrate is attached to a side of the elementsubstrate where the EL layer is formed under the atmosphere so that theEL layer is sealed. In a sealed state, a space between the elementsubstrate and the sealing substrate may be filled with an inert gas or aresin material. The sealing treatment chamber 190265 is provided with adispenser which draws a sealing material, a mechanical element such asan arm or a fixing stage which fixes the sealing substrate to face theelement substrate, a dispenser or a spin coater which fills the chamberwith a resin material, or the like.

FIG. 87 shows an internal structure of a deposition treatment chamber.The deposition treatment chamber is maintained in a reduced pressurestate. In FIG. 87, a space interposed between a top plate 190391 and abottom plate 190392 corresponds to an internal space of the chamber,which is maintained in a reduced pressure state.

One or a plurality of evaporation sources are provided in the treatmentchamber. This is because a plurality of evaporation sources arepreferably provided when a plurality of layers having differentcompositions are formed or when different materials are co-evaporated.In FIG. 87, evaporation sources 190381 a, 190381 b, and 190381 c areattached to an evaporation source holder 190380. The evaporation sourceholder 190380 is held by a multi-joint arm 190383. The multi-joint arm190383 allows the evaporation source holder 190380 to move within itsmovable range by stretching the joint. Alternatively, the evaporationsource holder 190380 may be provided with a distance sensor 190382 tomonitor a distance between the evaporation sources 190381 a to 190381 cand a substrate 190389 so that an optimal distance for evaporation iscontrolled. In this case, the multi-joint arm may be capable of movingtoward upper and lower directions (Z direction) as well.

The substrate 190389 is fixed by using a substrate stage 190386 and asubstrate chuck 190387 together. The substrate stage 190386 may have astructure where a heater is incorporated so that the substrate 190389can be heated. The substrate 190389 is fixed to the substrate stage190386 and transferred by the substrate chuck 190387. At the time ofevaporation, a shadow mask 190390 provided with an opening correspondingto an evaporation pattern can be used when needed. In this case, theshadow mask 190390 is arranged between the substrate 190389 and theevaporation sources 190381 a to 190381 c. The shadow mask 190390 adheresto the substrate 190389 or is fixed to the substrate 190389 with acertain interval therebetween by a mask chuck 190388. When alignment ofthe shadow mask 190390 is needed, the alignment is performed byarranging a camera in the treatment chamber and providing the mask chuck190388 with a positioning means which slightly moves in X-Y-θdirections.

Each of the evaporation sources 190381 a to 190381 c is provided with anevaporation material supply means which continuously supplies anevaporation material to the evaporation source. The evaporation materialsupply means includes material supply sources 190385 a, 190385 b, and190385 c, which are provided apart from the evaporation sources 190381a, 190381 b, and 190381 c, and a material supply pipe 190384 whichconnects the evaporation source and the material supply source.Typically, the material supply sources 190385 a to 190385 c are providedcorresponding to the evaporation sources 190381 a to 190381 c. In FIG.87, the material supply source 190385 a corresponds to the evaporationsource 190381 a, the material supply source 190385 b corresponds to theevaporation source 190381 b, and the material supply source 190385 ccorresponds to the evaporation source 190381 c.

As a method for supplying an evaporation material, an airflow transfermethod, an aerosol method, or the like can be employed. In an airflowtransfer method, impalpable powder of an evaporation material istransferred in airflow to the evaporation sources 190381 a to 190381 cby using an inert gas or the like. In an aerosol method, evaporation isperformed while material liquid in which an evaporation material isdissolved or dispersed in a solvent is transferred and aerosolized by anatomizer and the solvent in the aerosol is vaporized. In each case, theevaporation sources 190381 a to 190381 c are provided with a heatingmeans, and a film is formed over the substrate 190389 by vaporizing thetransferred evaporation material. In FIG. 87, the material supply pipe190384 can be bent flexibly and is formed of a thin pipe which hasenough rigidity not to be transformed even under reduced pressure.

When an airflow transfer method or an aerosol method is employed, filmformation may be performed in the deposition treatment chamber underatmospheric pressure or lower, and preferably under a reduced pressureof 133 to 13300 Pa. The pressure can be adjusted while an inert gas suchas helium, argon, neon, krypton, xenon, or nitrogen fills the depositiontreatment chamber or is supplied (and exhausted at the same time) to thedeposition treatment chamber. Note that an oxidizing atmosphere may beemployed by introducing a gas such as oxygen or nitrous oxide in thedeposition treatment chamber where an oxide film is formed. Alternately,a reducing atmosphere may be employed by introducing a gas such ashydrogen in the deposition treatment chamber where an organic materialis deposited.

As another method for supplying an evaporation material, a screw may beprovided in the material supply pipe 190384 to continuously push theevaporation material toward the evaporation source.

With this evaporation device, a film can be formed continuously withhigh uniformity even in the case of a large display panel. Since it isnot necessary to supply an evaporation material to the evaporationsource every time the evaporation material is run out, throughput can beimproved.

Note that although this embodiment mode is described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in each drawing in this embodiment modewith part of another embodiment mode.

Note that this embodiment mode shows examples of embodying, slightlytransforming, partially modifying, improving, describing in detail, orapplying the contents (or part of the contents) described in anotherembodiment mode, an example of related part thereof, or the like.Therefore, the contents described in another embodiment mode can befreely applied to, combined with, or replaced with this embodiment mode.

Embodiment Mode 18

In this embodiment mode, a structure of an EL element is described. Inparticular, a structure of an inorganic EL element is described.

An inorganic EL element is classified as either a dispersion typeinorganic EL element or a thin-film type inorganic EL element, dependingon its element structure. These elements differ in that the formerincludes an electroluminescent layer in which particles of alight-emitting material are dispersed in a binder, whereas the latterincludes an electroluminescent layer formed of a thin film of alight-emitting material. However, the former and the latter have incommon in that they need electrons accelerated by a high electric field.Note that mechanisms for obtaining light emission are donor-acceptorrecombination light emission which utilizes a donor level and anacceptor level; and localized light emission which utilizes inner-shellelectron transition of a metal ion. In general, donor-acceptorrecombination light emission is employed in dispersion type inorganic ELelements and localized light emission is employed in thin-film typeinorganic EL elements in many cases.

A light-emitting material includes a base material and an impurityelement to be a luminescence center. Light emission of various colorscan be obtained by changing the impurity element to be included. Thelight-emitting material can be formed using various methods, such as asolid phase method or a liquid phase method (a coprecipitation method).Further, a liquid phase method such as a spray pyrolysis method, adouble decomposition method, a method employing precursor pyrolysis, areverse micelle method, a method in which one or more of these methodsare combined with high-temperature baking, or a freeze-drying method, orthe like can be used.

A solid phase method is a method in which a base material and animpurity element or a compound containing an impurity element areweighed, mixed in a mortar, and heated and baked in an electric furnaceso as to be reacted; thus, the impurity element is included in the basematerial. The baking temperature is preferably 700 to 1500° C. This isbecause a solid-phase reaction does not proceed when the temperature istoo low, and the base material decomposes when the temperature is toohigh. Note that although the materials may be baked in powder form, theyare preferably baked in pellet form. Although a solid phase method needsa comparatively high temperature, it is a simple method, and thus hashigh productivity and is suitable for mass production.

A liquid phase method (a coprecipitation method) is a method in which abase material or a compound containing a base material, and an impurityelement or a compound containing an impurity element are reacted in asolution, dried, and then baked. Particles of a light-emitting materialare uniformly distributed, and the reaction can progress even when theparticles are small and the baking temperature is low.

As a base material to be used for a light-emitting material, sulfide,oxide, or nitride can be used. As sulfide, zinc sulfide (ZnS), cadmiumsulfide (CdS), calcium sulfide (CaS), yttrium sulfide (Y₂S₃), galliumsulfide (Ga₂S₃), strontium sulfide (SrS), barium sulfide (BaS), or thelike can be used, for example. As oxide, zinc oxide (ZnO), yttrium oxide(Y₂O₃), or the like can be used, for example. As nitride, aluminumnitride (AlN), gallium nitride (GaN), indium nitride (InN), or the likecan be used, for example. Further, zinc selenide (ZnSe), zinc telluride(ZnTe), or the like; or a ternary mixed crystal such as calcium galliumsulfide (CaGa₂S₄), strontium gallium sulfide (SrGa₂S₄), or bariumgallium sulfide (BaGa₂S₄) may be used.

As a luminescence center for localized light emission, manganese (Mn),copper (Cu), samarium (Sm), terbium (Th), erbium (Er), thulium (Tm),europium (Eu), cerium (Ce), praseodymium (Pr), or the like can be used.Note that a halogen element such as fluorine (F) or chlorine (Cl) may beadded for charge compensation.

On the other hand, as a luminescence center for donor-acceptorrecombination light emission, a light-emitting material including afirst impurity element forming a donor level and a second impurityelement forming an acceptor level can be used. As the first impurityelement, fluorine (F), chlorine (Cl), aluminum (Al), or the like can beused, for example. As the second impurity element, copper (Cu), silver(Ag), or the like can be used, for example.

When the light-emitting material for donor-acceptor recombination lightemission is synthesized by a solid phase method, a base material, thefirst impurity element or a compound containing the first impurityelement, and the second impurity element or a compound containing thesecond impurity element are weighed, mixed in a mortar, and heated andbaked in an electric furnace. As the base material, the aforementionedbase material can be used. As the first impurity element or the compoundcontaining the first impurity element, fluorine (F), chlorine (Cl),aluminum sulfide (Al₂S₃), or the like can be used, for example. As thesecond impurity element or the compound containing the second impurityelement, copper (Cu), silver (Ag), copper sulfide (Cu₂S), silver sulfide(Ag₂S), or the like can be used, for example. The baking temperature ispreferably 700 to 1500° C. This is because a solid-phase reaction doesnot proceed when the temperature is too low, and the base materialdecomposes when the temperature is too high. Note that although thematerials may be baked in powder form, they are preferably baked inpellet form.

As the impurity element in the case of using a solid phase reaction,compounds including the first impurity element and the second impurityelement may be used in combination. In this case, the impurity elementsare easily diffused, and the solid phase reaction proceeds readily, sothat a uniform light-emitting material can be obtained. Further, sincean unnecessary impurity element is not included, a light-emittingmaterial with high purity can be obtained. As the compound including thefirst impurity element and the second impurity element, copper chloride(CuCl), silver chloride (AgCl), or the like can be used, for example.

Note that the concentration of these impurity elements is in the rangeof 0.01 to 10 atomic percent, and is preferably in the range of 0.05 to5 atomic percent with respect to the base material.

In the case of a thin-film type inorganic EL element, anelectroluminescent layer includes the aforementioned light-emittingmaterial, and can be formed using a physical vapor deposition (PVD)method such as a sputtering method or a vacuum evaporation method, forexample, a resistance heating evaporation method or an electron beamevaporation (EB evaporation) method, a chemical vapor deposition (CVD)method such as a metal organic CVD method or a low-pressure hydridetransport CVD method, an atomic layer epitaxy (ALE) method, or the like.FIGS. 88A to 88C each show an example of a thin-film type inorganic ELelement which can be used as the light-emitting element. In FIGS. 88A to88C, a light-emitting element includes a first electrode layer 120100,an electroluminescent layer 120102, and a second electrode layer 120103.

The light-emitting elements shown in FIGS. 88B and 88C each have astructure where an insulating film is provided between the electrodelayer and the electroluminescent layer in the light-emitting element inFIG. 88A. The light-emitting element shown in FIG. 88B includes aninsulating film 120104 between the first electrode layer 120100 and theelectroluminescent layer 120102. The light-emitting element shown inFIG. 88C includes an insulating film 120105 between the first electrodelayer 120100 and the electroluminescent layer 120102, and an insulatingfilm 120106 between the second electrode layer 120103 and theelectroluminescent layer 120102.

In such a manner, the insulating film may be provided between theelectroluminescent layer and one of the electrode layers interposing theelectroluminescent layer, or may be provided between theelectroluminescent layer and each of the electrode layers interposingthe electroluminescent layer. The insulating film may be a single layeror stacked layers including a plurality of layers.

Note that the insulating film 120104 is provided in contact with thefirst electrode layer 120100 in FIG. 88B; however, the insulating film120104 may be provided in contact with the second electrode layer 120103by reversing the order of the insulating film and the electroluminescentlayer.

In the case of a dispersion type inorganic EL, a film-shapedelectroluminescent layer is formed by dispersing particulatelight-emitting materials in a binder. When particles with a desired sizecannot be sufficiently obtained by a method of forming thelight-emitting material, the light-emitting materials may be processedinto particles by being crushed in a mortar or the like. The binder is asubstance for fixing the particulate light-emitting material in adispersed state and maintaining the shape as the electroluminescentlayer. The light-emitting material is uniformly dispersed in theelectroluminescent layer and fixed by the binder.

In the case of a dispersion type inorganic EL, as a method of formingthe electroluminescent layer, a droplet discharging method by which theelectroluminescent layer can be selectively formed, a printing method(such as screen printing or offset printing), a coating method such as aspin coating method, a dipping method, a dispenser method, or the likecan be used. The thickness of the electroluminescent layer is notparticularly limited, but preferably in the range of 10 to 1000 nm. Inthe electroluminescent layer including the light-emitting material andthe binder, a ratio of the light-emitting material is preferably 50 wt %or more and 80 wt % or less.

FIGS. 89A to 89C each show an example of a dispersion type inorganic ELelement which can be used as the light-emitting element. Alight-emitting element in FIG. 89A has a stacked-layer structure of afirst electrode layer 120200, an electroluminescent layer 120202, and asecond electrode layer 120203. The electroluminescent layer 120202includes a light-emitting material 120201 held by a binder.

An insulating material can be used for the binder. As the insulatingmaterial, an organic material or an inorganic material can be used.Alternatively, a mixed material containing an organic material and aninorganic material may be used. As the organic insulating material, apolymer having a comparatively high dielectric constant, such as acyanoethyl cellulose based resin, or a resin such as polyethylene,polypropylene, a polystyrene based resin, a silicone resin, an epoxyresin, or vinylidene fluoride can be used. Alternatively, aheat-resistant polymer such as aromatic polyamide or polybenzimidazole,or a siloxane resin may be used. Note that a siloxane resin correspondsto a resin having Si—O—Si bonds. Siloxane includes a skeleton structureof a bond of silicon (Si) and oxygen (O). As a substituent, an organicgroup containing at least hydrogen (such as an alkyl group or aromatichydrocarbon) is used. Alternatively, a fluoro group, or a fluoro groupand an organic group containing at least hydrogen may be used as asubstituent. Further alternately, a resin material, for example, a vinylresin such as polyvinyl alcohol or polyvinylbutyral, a phenol resin, anovolac resin, an acrylic resin, a melamine resin, an urethane resin, oran oxazole resin (polybenzoxazole) may be used. A dielectric constantcan be adjusted by appropriately mixing these resins with fine particleshaving a high dielectric constant, such as barium titanate (BaTiO₃) orstrontium titanate (SrTiO₃).

The inorganic insulating material included in the binder can be formedusing silicon oxide (SiO_(x)), silicon nitride (SiN_(x)), siliconcontaining oxygen and nitrogen, aluminum nitride (AlN), aluminumcontaining oxygen and nitrogen, aluminum oxide (Al₂O₃) containing oxygenand nitrogen, titanium oxide (TiO₂), BaTiO₃, SrTiO₃, lead titanate(PbTiO₃), potassium niobate (KNbO₃), lead niobate (PbNbO₃), tantalumoxide (Ta₂O₅), barium tantalite (BaTa₂O₆), lithium tantalite (LiTaO₃),yttrium oxide (Y₂O₃), zirconium oxide (ZrO₂), ZnS, or a substancecontaining another inorganic insulating material. When an inorganicmaterial having a high dielectric constant is included in the organicmaterial (by addition or the like), the dielectric constant of theelectroluminescent layer formed of the light-emitting material and thebinder can be more effectively controlled, and the dielectric constantcan be further increased.

In a manufacturing step, the light-emitting material is dispersed in asolution containing the binder. As a solvent for the solution containingthe binder, it is acceptable as long as a solvent dissolves a bindermaterial and can make a solution having a viscosity suitable for amethod of forming the electroluminescent layer (various wet processes)and for desired film thickness. For example, an organic solvent or thelike can be used as the solvent. When a siloxane resin is used as thebinder, propylene glycol monomethyl ether, propylene glycol monomethylether acetate (also referred to as PGMEA), 3-methoxy-3-methyl-1-butanol(also referred to as MMB), or the like can be used as the solvent.

The light-emitting elements shown in FIGS. 89B and 89C each have astructure where an insulating film is provided between the electrodelayer and the electroluminescent layer in the light-emitting element inFIG. 89A. The light-emitting element shown in FIG. 89B includes aninsulating film 120204 between the first electrode layer 120200 and theelectroluminescent layer 120202. The light-emitting element shown inFIG. 89C includes an insulating film 120205 between the first electrodelayer 120200 and the electroluminescent layer 120202, and an insulatingfilm 120206 between the second electrode layer 120203 and theelectroluminescent layer 120202. In such a manner, the insulating filmmay be provided between the electroluminescent layer and one of theelectrode layers interposing the electroluminescent layer, or may beprovided between the electroluminescent layer and each of the electrodelayers interposing the electroluminescent layer. The insulating film maybe a single layer or stacked layers including a plurality of layers.

Although the insulating film 120204 is provided in contact with thefirst electrode layer 120200 in FIG. 89B, the insulating film 120204 maybe provided in contact with the second electrode layer 120203 byreversing the order of the insulating film and the electroluminescentlayer.

A material used for an insulating film such as the insulating film120104 in FIG. 88B and the insulating film 120204 in FIG. 89B preferablyhas high withstand voltage and dense film quality. Further, the materialpreferably has a high dielectric constant. For example, silicon oxide(SiO₂), yttrium oxide (Y₂O₃), titanium oxide (TiO₂), aluminum oxide(Al₂O₃), hafnium oxide (HfO₂), tantalum oxide (Ta₂O₅), barium titanate(BaTiO₃), strontium titanate (SrTiO₃), lead titanate (PbTiO₃), siliconnitride (Si₃N₄), zirconium oxide (ZrO₂), or the like; or a mixed film ofthese materials or a stacked-layer film including two or more of thosematerials can be used. The insulating film can be formed by sputtering,evaporation, CVD, or the like. The insulating film may be formed bydispersing particles of the insulating material in a binder. A bindermaterial may be formed using a material and a method similar to those ofthe binder contained in the electroluminescent layer. The thickness ofthe insulating film is not particularly limited, but preferably in therange of 10 to 1000 nm.

Note that the light-emitting element can emit light when voltage isapplied between the pair of electrode layers interposing theelectroluminescent layer. The light-emitting element can operate with DCdrive or AC drive.

Note that although this embodiment mode is described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in each drawing in this embodiment modewith part of another embodiment mode.

Note that this embodiment mode shows examples of embodying, slightlytransforming, partially modifying, improving, describing in detail, orapplying the contents (or part of the contents) described in anotherembodiment mode, an example of related part thereof, or the like.Therefore, the contents described in another embodiment mode can befreely applied to, combined with, or replaced with this embodiment mode.

Embodiment Mode 19

In this embodiment mode, an example of a display device is described. Inparticular, the case where optical treatment is performed is described.

A rear projection display device 130100 in FIGS. 90A and 90B is providedwith a projector unit 130111, a mirror 130112, and a screen panel130101. The rear projection display device 130100 may also be providedwith a speaker 130102 and operation switches 130104. The projector unit130111 is provided at a lower portion of a housing 130110 of the rearprojection display device 130100, and projects incident light whichprojects an image based on a video signal to the mirror 130112. The rearprojection display device 130100 displays an image projected from a rearsurface of the screen panel 130101.

FIG. 91 shows a front projection display device 130200. The frontprojection display device 130200 is provided with the projector unit130111 and a projection optical system 130201. The projection opticalsystem 130201 projects an image to a screen or the like provided at thefront.

Hereinafter, a structure of the projector unit 130111 which is appliedto the rear projection display device 130100 in FIGS. 90A and 90B andthe front projection display device 130200 in FIG. 91 is described.

FIG. 92 shows a structure example of the projector unit 130111. Theprojector unit 130111 is provided with a light source unit 130301 and amodulation unit 130304. The light source unit 130301 is provided with alight source optical system 130303 including lenses and a light sourcelamp 130302. The light source lamp 130302 is stored in a housing so thatstray light is not scattered. As the light source lamp 130302, ahigh-pressure mercury lamp or a xenon lamp, for example, which can emita large amount of light, is used. The light source optical system 130303is provided with an optical lens, a film having a function of polarizinglight, a film for adjusting phase difference, an IR film, or the like asappropriate. The light source unit 130301 is provided so that emittedlight is incident on the modulation unit 130304. The modulation unit130304 is provided with a plurality of display panels 130308, a colorfilter, a dichroic mirror 130305, a total reflection mirror 130306, aprism 130309, and a projection optical system 130310. Light emitted fromthe light source unit 130301 is split into a plurality of optical pathsby the dichroic mirror 130305.

The display panel 130308 and a color filter which transmits light with apredetermined wavelength or wavelength range are provided in eachoptical path. The transmissive display panel 130308 modulatestransmitted light based on a video signal. Light of each colortransmitted through the display panel 130308 is incident on the prism130309, and an image is displayed on a screen through the projectionoptical system 130310. Note that a Fresnel lens may be provided betweenthe mirror and the screen. Then, projected light which is projected bythe projector unit 130111 and reflected by the mirror is converted intogenerally parallel light by the Fresnel lens and projected on thescreen.

FIG. 93 shows the projector unit 130111 provided with reflective displaypanels 130407, 130408, and 130409.

The projector unit 130111 shown in FIG. 93 is provided with the lightsource unit 130301 and a modulation unit 130400. The light source unit130301 may have a structure similar to that of FIG. 92. Light from thelight source unit 130301 is split into a plurality of optical paths bydichroic mirrors 130401 and 130402 and a total reflection mirror 130403to be incident on polarization beam splitters 130404, 130405, and130406. The polarization beam splitters 130404, 130405, and 130406 areprovided corresponding to the reflective display panels 130407, 130408,and 130409 which correspond to respective colors. The reflective displaypanels 130407, 130408, and 130409 modulate reflected light based on avideo signal. Light of respective colors which is reflected by thereflective display panels 130407, 130408, and 130409 is incident on theprism 130410 to be synthesized, and projected through a projectionoptical system 130411.

Among light emitted from the light source unit 130301, only light in ared wavelength region is transmitted through the dichroic mirror 130401and light in green and blue wavelength regions is reflected by thedichroic mirror 130401. Further, only the light in the green wavelengthregion is reflected by the dichroic mirror 130402. The light in the redwavelength region, which is transmitted through the dichroic mirror130401, is reflected by the total reflection mirror 130403 and incidenton the polarization beam splitter 130404. The light in the bluewavelength region is incident on the polarization beam splitter 130405.The light in the green wavelength region is incident on the polarizationbeam splitter 130406. The polarization beam splitters 130404, 130405,and 130406 have a function of splitting incident light into p-polarizedlight and s-polarized light and a function of transmitting onlyp-polarized light. The reflective display panels 130407, 130408, and130409 polarize incident light based on a video signal.

Only s-polarized light corresponding to respective colors is incident onthe reflective display panels 130407, 130408, and 130409 correspondingto respective colors. Note that the reflective display panels 130407,130408, and 130409 may be liquid crystal panels. In this case, theliquid crystal panel operates in an electrically controlledbirefringence (ECB) mode. Liquid crystal molecules are verticallyaligned with respect to a substrate at a certain angle. Accordingly, inthe reflective display panels 130407, 130408, and 130409, when a pixelis in an off state, display molecules are aligned so as to reflectincident light without changing a polarization state of the incidentlight. When the pixel is in an on state, alignment of the displaymolecules is changed, and the polarization state of the incident lightis changed.

The projector unit 130111 in FIG. 93 can be applied to the rearprojection display device 130100 in FIGS. 90A and 90B and the frontprojection display device 130200 in FIG. 91.

FIGS. 94A to 94C show single-panel type projector units. The projectorunit 130111 shown in FIG. 94A is provided with the light source unit130301, a display panel 130507, a projection optical system 130511, anda retardation plate 130504. The projection optical system 130511includes one or a plurality of lenses. The display panel 130507 may beprovided with a color filter.

FIG. 94B shows a structure of the projector unit 130111 operating in afield sequential mode. A field sequential mode refers to a mode in whichcolor display is performed by light of respective colors such as red,green, and blue sequentially incident on a display panel with a timelag, without a color filter. High-definition image can be displayedparticularly by combination with a display panel with high responsespeed to change in input signal. In FIG. 94B, a rotating color filterplate 130505 including a plurality of color filters with red, green,blue, or the like is provided between the light source unit 130301 and adisplay panel 130508.

FIG. 94C shows a structure of the projector unit 130111 with a colorseparation method using a micro lens, as a color display method. Thismethod refers to a method in which color display is realized byproviding a micro lens array 130506 on a light incident side of adisplay panel 130509 and emitting light of each color from eachdirection. The projector unit 130111 employing this method has littleloss of light due to a color filter, so that light from the light sourceunit 130301 can be efficiently utilized. The projector unit 130111 shownin FIG. 94C is provided with dichroic mirrors 130501, 130502, and 130503so that light of each color is lit to the display panel 130509 from eachdirection.

Note that although this embodiment mode is described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in each drawing in this embodiment modewith part of another embodiment mode.

Note that this embodiment mode shows examples of embodying, slightlytransforming, partially modifying, improving, describing in detail, orapplying the contents (or part of the contents) described in anotherembodiment mode, an example of related part thereof, or the like.Therefore, the contents described in another embodiment mode can befreely applied to, combined with, or replaced with this embodiment mode.

Embodiment Mode 20

In this embodiment mode, examples of electronic devices according to thepresent invention are described.

FIG. 95 shows a display panel module combining a display panel 900101and a circuit board 900111. The display panel 900101 includes a pixelportion 900102, a scan line driver circuit 900103, and a signal linedriver circuit 900104. The circuit board 900111 is provided with acontrol circuit 900112, a signal dividing circuit 900113, and the like,for example. The display panel 900101 and the circuit board 900111 areconnected by a connection wiring 900114. An FPC or the like can be usedfor the connection wiring.

In the display panel 900101, the pixel portion 900102 and part ofperipheral driver circuits (a driver circuit having a low operationfrequency among a plurality of driver circuits) may be formed over thesame substrate by using transistors, and another part of the peripheraldriver circuits (a driver circuit having a high operation frequencyamong the plurality of driver circuits) may be formed over an IC chip.Then, the IC chip may be mounted on the display panel 900101 by COG(chip on glass) or the like. Thus, the area of the circuit board 900111can be reduced, and a small display device can be obtained.Alternatively, the IC chip may be mounted on the display panel 900101 byusing TAB (tape automated bonding) or a printed wiring board. Thus, thearea of the display panel 900101 can be reduced, and a display devicewith a narrower frame can be obtained.

For example, in order to reduce power consumption, a pixel portion maybe formed over a glass substrate by using transistors, and allperipheral circuits may be formed over an IC chip. Then, the IC chip maybe mounted on a display panel by COG or TAB.

A television receiver can be completed with the display panel moduleshown in FIG. 95. FIG. 96 is a block diagram showing a main structure ofa television receiver. A tuner 900201 receives a video signal and anaudio signal. The video signals are processed by a video signalamplifier circuit 900202; a video signal processing circuit 900203 whichconverts a signal output from the video signal amplifier circuit 900202into a color signal corresponding to each color of red, green, and blue;and a control circuit 900212 which converts the video signal into aninput specification of a driver circuit. The control circuit 900212outputs signals to each of the scan line side and the signal line side.When digital driving is performed, a structure may be employed in whicha signal dividing circuit 900213 is provided on the signal line side andan input digital signal is divided into m signals (m is a positiveinteger) to be supplied.

Among the signals received by the tuner 900201, an audio signal istransmitted to an audio signal amplifier circuit 900205, and an outputthereof is supplied to a speaker 900207 through an audio signalprocessing circuit 900206. A control circuit 900208 receives controlinformation on receiving station (receiving frequency) and volume froman input portion 900209 and transmits a signal to the tuner 900201 orthe audio signal processing circuit 900206.

FIG. 97A shows a television receiver incorporated with a display panelmodule which is different from FIG. 96. In FIG. 97A, a display screen900302 stored in a housing 900301 is formed using the display panelmodule. Note that speakers 900303, an operation switch 900304, an inputmeans 900305, a sensor 900306 (having a function of measuring force,displacement, position, speed, acceleration, angular velocity, rotationnumber, distance, light, liquid, magnetism, temperature, chemicalreaction, sound, time, hardness, electric field, current, voltage,electric power, radial ray, flow rate, humidity, gradient, vibration,smell, or infrared ray), a microphone 900307, and the like may beprovided as appropriate.

FIG. 97B shows a television receiver in which only a display can becarried wirelessly. A battery and a signal receiver are incorporated ina housing 900312. By the battery, a display portion 900313, a speakerportion 900317, a sensor 900319 (having a function of measuring force,displacement, position, speed, acceleration, angular velocity, rotationnumber, distance, light, liquid, magnetism, temperature, chemicalreaction, sound, time, hardness, electric field, current, voltage,electric power, radial ray, flow rate, humidity, gradient, vibration,smell, or infrared ray), and a microphone 900320 are driven. The batterycan be repeatedly charged by a charger 900310. The charger 900310 whichis capable of transmitting and receiving a video signal can transmit thevideo signal to the signal receiver of the display. The device shown inFIG. 97B is controlled by an operation key 900316. Alternatively, thedevice shown in FIG. 97B can transmit a signal to the charger 900310 byoperating the operation key 900316. That is, the device may be an imageand audio interactive communication device. Further alternatively, byoperating the operation key 900316, a signal is transmitted to thecharger 900310 from the housing 900312, and another electronic device ismade to receive a signal which can be transmitted from the charger900310; thus, the device shown in FIG. 97B can control communication ofanother electronic device. That is, the device may be a general-purposeremote control device. Note that an input means 900318 or the like maybe provided as appropriate. Note that the contents (or part of thecontents) described in each drawing of this embodiment mode can beapplied to the display portion 900313.

FIG. 98A shows a module combining a display panel 900401 and a printedwiring board 900402. The display panel 900401 may be provided with apixel portion 900403 including a plurality of pixels, a first scan linedriver circuit 900404, a second scan line driver circuit 900405, and asignal line driver circuit 900406 which supplies a video signal to aselected pixel.

The printed wiring board 900402 is provided with a controller 900407, acentral processing unit (CPU) 900408, a memory 900409, a power supplycircuit 900410, an audio processing circuit 900411, atransmitting/receiving circuit 900412, and the like. The printed wiringboard 900402 and the display panel 900401 are connected by a flexibleprinted circuit (FPC) 900413. The flexible printed circuit (FPC) 900413may be provided with a capacitor, a buffer circuit, or the like so as toprevent noise on power supply voltage or a signal, and increase in risetime of a signal. Note that the controller 900407, the audio processingcircuit 900411, the memory 900409, the central processing unit (CPU)900408, the power supply circuit 900410, or the like can be mounted tothe display panel 900401 by using a COG (chip on glass) method. By usinga COG method, the size of the printed wiring board 900402 can bereduced.

Various control signals are input and output through an interface (I/F)portion 900414 provided for the printed wiring board 900402. An antennaport 900415 for transmitting and receiving a signal to/from an antennais provided for the printed wiring board 900402.

FIG. 98B is a block diagram of the module shown in FIG. 98A. The moduleincludes a VRAM 900416, a DRAM 900417, a flash memory 900418, and thelike as the memory 900409. The VRAM 900416 stores data on an imagedisplayed on a panel, the DRAM 900417 stores video data or audio data,and the flash memory 900418 stores various programs.

The power supply circuit 900410 supplies electric power for operatingthe display panel 900401, the controller 900407, the central processingunit (CPU) 900408, the audio processing circuit 900411, the memory900409, and the transmitting/receiving circuit 900412. Note that thepower supply circuit 900410 may be provided with a current sourcedepending on a panel specification.

The central processing unit (CPU) 900408 includes a control signalgeneration circuit 900420, a decoder 900421, a register 900422, anarithmetic circuit 900423, a RAM 900424, an interface (I/F) portion900419 for the central processing unit (CPU) 900408, and the like.Various signals input to the central processing unit (CPU) 900408 viathe interface (I/F) portion 900414 are once stored in the register900422, and subsequently input to the arithmetic circuit 900423, thedecoder 900421, and the like. The arithmetic circuit 900423 performsoperation based on the signal input thereto so as to designate alocation to which various instructions are sent. On the other hand, thesignal input to the decoder 900421 is decoded and input to the controlsignal generation circuit 900420. The control signal generation circuit900420 generates a signal including various instructions based on thesignal input thereto, and transmits the signal to the locationdesignated by the arithmetic circuit 900423, specifically the memory900409, the transmitting/receiving circuit 900412, the audio processingcircuit 900411, and the controller 900407, for example.

The memory 900409, the transmitting/receiving circuit 900412, the audioprocessing circuit 900411, and the controller 900407 operate inaccordance with respective instructions. Hereinafter, the operation isbriefly described.

A signal input from an input means 900425 is transmitted via theinterface (I/F) portion 900414 to the central processing unit (CPU)900408 mounted to the printed wiring board 900402. The control signalgeneration circuit 900420 converts image data stored in the VRAM 900416into a predetermined format depending on the signal transmitted from theinput means 900425 such as a pointing device or a keyboard, andtransmits the converted data to the controller 900407.

The controller 900407 performs data processing of the signal includingthe image data transmitted from the central processing unit (CPU) 900408in accordance with the panel specification, and supplies the signal tothe display panel 900401. The controller 900407 generates an Hsyncsignal, a Vsync signal, a clock signal CLK, alternating voltage (ACCont), and a switching signal L/R based on power supply voltage inputfrom the power supply circuit 900410 or various signals input from thecentral processing unit (CPU) 900408, and supplies the signals to thedisplay panel 900401.

The transmitting/receiving circuit 900412 processes a signal which is tobe transmitted and received as an electric wave by an antenna 900428.Specifically, the transmitting/receiving circuit 900412 may include ahigh-frequency circuit such as an isolator, a band pass filter, a VCO(voltage controlled oscillator), an LPF (low pass filter), a coupler, ora balun. A signal including audio information among signals transmittedand received by the transmitting/receiving circuit 900412 is transmittedto the audio processing circuit 900411 in accordance with an instructionfrom the central processing unit (CPU) 900408.

The signal including the audio information which is transmitted inaccordance with the instruction from the central processing unit (CPU)900408 is demodulated into an audio signal by the audio processingcircuit 900411 and transmitted to a speaker 900427. An audio signaltransmitted from a microphone 900426 is modulated by the audioprocessing circuit 900411 and transmitted to the transmitting/receivingcircuit 900412 in accordance with an instruction from the centralprocessing unit (CPU) 900408.

The controller 900407, the central processing unit (CPU) 900408, thepower supply circuit 900410, the audio processing circuit 900411, andthe memory 900409 can be mounted as a package of this embodiment mode.

It is needless to say that this embodiment mode is not limited to atelevision receiver and can be applied to various uses, such as amonitor of a personal computer, and especially as a large display mediumsuch as an information display board at the train station, the airport,or the like, or an advertisement display board on the street.

Next, a structure example of a mobile phone according to the presentinvention is described with reference to FIG. 99.

A display panel 900501 is detachably incorporated in a housing 900530.The shape or the size of the housing 900530 can be changed asappropriate in accordance with the size of the display panel 900501. Thehousing 900530 to which the display panel 900501 is fixed is fitted in aprinted wiring board 900531 to be assembled as a module.

The display panel 900501 is connected to the printed wiring board 900531through an FPC 900513. The printed wiring board 900531 is provided witha speaker 900532, a microphone 900533, a transmitting/receiving circuit900534, a signal processing circuit 900535 including a CPU, acontroller, and the like, and a sensor 900541 (having a function ofmeasuring force, displacement, position, speed, acceleration, angularvelocity, rotation number, distance, light, liquid, magnetism,temperature, chemical reaction, sound, time, hardness, electric field,current, voltage, electric power, radial ray, flow rate, humidity,gradient, vibration, smell, or infrared ray). Such a module, an inputmeans 900536, and a battery 900537 are combined and stored in a housing900539. A pixel portion of the display panel 900501 is provided to beseen from an opening window formed in the housing 900539.

In the display panel 900501, the pixel portion and part of peripheraldriver circuits (a driver circuit having a low operation frequency amonga plurality of driver circuits) may be formed over the same substrate byusing transistors, and another part of the peripheral driver circuits (adriver circuit having a high operation frequency among the plurality ofdriver circuits) may be formed over an IC chip. Then, the IC chip may bemounted on the display panel 900501 by COG (chip on glass).Alternatively, the IC chip may be connected to a glass substrate byusing TAB (tape automated bonding) or a printed wiring board. With sucha structure, power consumption of a display device can be reduced, andoperation time of the mobile phone per charge can be extended. Further,reduction in cost of the mobile phone can be realized.

The mobile phone shown in FIG. 99 has various functions such as, but notlimited to, a function of displaying various kinds of information (e.g.,a still image, a moving image, and a text image); a function ofdisplaying a calendar, a date, the time, and the like on a displayportion; a function of operating or editing the information displayed onthe display portion; a function of controlling processing by variouskinds of software (programs); a function of wireless communication; afunction of communicating with another mobile phone, a fixed phone, oran audio communication device by using the wireless communicationfunction; a function of connecting with various computer networks byusing the wireless communication function; a function of transmitting orreceiving various kinds of data by using the wireless communicationfunction; a function of operating a vibrator in accordance with incomingcall, reception of data, or an alarm; and a function of generating asound in accordance with incoming call, reception of data, or an alarm.

In a mobile phone shown in FIG. 100, a main body (A) 900601 providedwith operation switches 900604, a microphone 900605, and the like isconnected to a main body (B) 900602 provided with a display panel (A)900608, a display panel (B) 900609, a speaker 900606, a sensor 900611(having a function of measuring force, displacement, position, speed,acceleration, angular velocity, rotation number, distance, light,liquid, magnetism, temperature, chemical reaction, sound, time,hardness, electric field, current, voltage, electric power, radial ray,flow rate, humidity, gradient, vibration, smell, or infrared ray), aninput means 900612, and the like by using a hinge 900610 so that themobile phone can be opened and closed. The display panel (A) 900608 andthe display panel (B) 900609 are placed in a housing 900603 of the mainbody (B) 900602 together with a circuit board 900607. Each of pixelportions of the display panel (A) 900608 and the display panel (B)900609 is arranged to be seen from an opening window formed in thehousing 900603.

Specifications of the display panel (A) 900608 and the display panel (B)900609, such as the number of pixels, can be set as appropriate inaccordance with functions of a mobile phone 900600. For example, thedisplay panel (A) 900608 can be used as a main screen and the displaypanel (B) 900609 can be used as a sub-screen.

A mobile phone according to this embodiment mode can be changed invarious modes depending on functions or applications thereof. Forexample, it may be a camera-equipped mobile phone by incorporating animaging element in a portion of the hinge 900610. When the operationswitches 900604, the display panel (A) 900608, and the display panel (B)900609 are placed in one housing, the aforementioned effects can beobtained. Further, a similar effect can be obtained when the structureof this embodiment mode is applied to an information display terminalequipped with a plurality of display portions.

The mobile phone in FIG. 100 has various functions such as, but notlimited to, a function of displaying various kinds of information (e.g.,a still image, a moving image, and a text image); a function ofdisplaying a calendar, a date, the time, and the like on a displayportion; a function of operating or editing the information displayed onthe display portion; a function of controlling processing by variouskinds of software (programs); a function of wireless communication; afunction of communicating with another mobile phone, a fixed phone, oran audio communication device by using the wireless communicationfunction; a function of connecting with various computer networks byusing the wireless communication function; a function of transmitting orreceiving various kinds of data by using the wireless communicationfunction; a function of operating a vibrator in accordance with incomingcall, reception of data, or an alarm; and a function of generating asound in accordance with incoming call, reception of data, or an alarm.

The contents (or part of the contents) described in each drawing in thisembodiment mode can be applied to various electronic devices.Specifically, the present invention can be applied to a display portionof an electronic device. Examples of such electronic devices includecameras such as a video camera and a digital camera, a goggle-typedisplay, a navigation system, an audio reproducing device (such as caraudio components and audio components), a computer, a game machine, aportable information terminal (such as a mobile computer, a mobilephone, a mobile game machine, and an e-book reader), and an imagereproducing device provided with a recording medium (specifically, adevice which reproduces a recording medium such as a digital versatiledisc (DVD) and has a display for displaying the reproduced image).

FIG. 101A shows a display, which includes a housing 900711, a supportbase 900712, a display portion 900713, an input means 900714, a sensor900715 (having a function of measuring force, displacement, position,speed, acceleration, angular velocity, rotation number, distance, light,liquid, magnetism, temperature, chemical reaction, sound, time,hardness, electric field, current, voltage, electric power, radial ray,flow rate, humidity, gradient, vibration, smell, or infrared ray), amicrophone 900716, a speaker 900717, operation keys 900718, an LED lamp900719, and the like. The display shown in FIG. 101A can have variousfunctions such as, but not limited to, a function of displaying variouskinds of information (e.g., a still image, a moving image, and a textimage) on the display portion.

FIG. 101B shows a camera, which includes a main body 900731, a displayportion 900732, an image receiving portion 900733, operation keys900734, an external connection port 900735, a shutter button 900736, aninput means 900737, a sensor 900738 (having a function of measuringforce, displacement, position, speed, acceleration, angular velocity,rotation number, distance, light, liquid, magnetism, temperature,chemical reaction, sound, time, hardness, electric field, current,voltage, electric power, radial ray, flow rate, humidity, gradient,vibration, smell, or infrared ray), a microphone 900739, a speaker900740, an LED lamp 900741, and the like. The camera shown in FIG. 101Bcan have various functions such as, but not limited to, a function ofphotographing a still image and a moving image; a function ofautomatically adjusting the photographed image (the still image or themoving image); a function of storing the photographed image in arecording medium (provided externally or incorporated in the camera);and a function of displaying the photographed image on the displayportion.

FIG. 101C shows a computer, which includes a main body 900751, a housing900752, a display portion 900753, a keyboard 900754, an externalconnection port 900755, a pointing device 900756, an input means 900757,a sensor 900758 (having a function of measuring force, displacement,position, speed, acceleration, angular velocity, rotation number,distance, light, liquid, magnetism, temperature, chemical reaction,sound, time, hardness, electric field, current, voltage, electric power,radial ray, flow rate, humidity, gradient, vibration, smell, or infraredray), a microphone 900759, a speaker 900760, an LED lamp 900761, areader/writer 900762, and the like. The computer shown in FIG. 101C canhave various functions such as, but not limited to, a function ofdisplaying various kinds of information (e.g., a still image, a movingimage, and a text image) on the display portion; a function ofcontrolling processing by various kinds of software (programs); acommunication function such as wireless communication or wirecommunication; a function of connecting with various computer networksby using the communication function; and a function of transmitting orreceiving various kinds of data by using the communication function.

FIG. 108A shows a mobile computer, which includes a main body 901411, adisplay portion 901412, a switch 901413, operation keys 901414, aninfrared port 901415, an input means 901416, a sensor 901417 (having afunction of measuring force, displacement, position, speed,acceleration, angular velocity, rotation number, distance, light,liquid, magnetism, temperature, chemical reaction, sound, time,hardness, electric field, current, voltage, electric power, radial ray,flow rate, humidity, gradient, vibration, smell, or infrared ray), amicrophone 901418, a speaker 901419, an LED lamp 901420, and the like.The mobile computer shown in FIG. 108A can have various functions suchas, but not limited to, a function of displaying various kinds ofinformation (e.g., a still image, a moving image, and a text image) onthe display portion; a touch panel function provided on the displayportion; a function of displaying a calendar, a date, the time, and thelike on the display portion; a function of controlling processing byvarious kinds of software (programs); a function of wirelesscommunication; a function of connecting with various computer networksby using the wireless communication function; and a function oftransmitting or receiving various kinds of data by using the wirelesscommunication function.

FIG. 108B shows a portable image reproducing device provided with arecording medium (e.g., a DVD reproducing device), which includes a mainbody 901431, a housing 901432, a display portion A 901433, a displayportion B 901434, a recording medium (e.g., DVD) reading portion 901435,operation keys 901436, a speaker portion 901437, an input means 901438,a sensor 901439 (having a function of measuring force, displacement,position, speed, acceleration, angular velocity, rotation number,distance, light, liquid, magnetism, temperature, chemical reaction,sound, time, hardness, electric field, current, voltage, electric power,radial ray, flow rate, humidity, gradient, vibration, smell, or infraredray), a microphone 901440, an LED lamp 901441, and the like. The displayportion A 901433 can mainly display image information, and the displayportion B 901434 can mainly display text information.

FIG. 108C shows a goggle-type display, which includes a main body901451, a display portion 901452, an earphone 901453, a support portion901454, an input means 901455, a sensor 901456 (having a function ofmeasuring force, displacement, position, speed, acceleration, angularvelocity, rotation number, distance, light, liquid, magnetism,temperature, chemical reaction, sound, time, hardness, electric field,current, voltage, electric power, radial ray, flow rate, humidity,gradient, vibration, smell, or infrared ray), a microphone 901457, aspeaker 901458, an LED lamp 901459, and the like. The goggle-typedisplay shown in FIG. 108C can have various functions such as, but notlimited to, a function of displaying an image (e.g., a still image, amoving image, and a text image) which is externally obtained on thedisplay portion.

FIG. 109A shows a portable game machine, which includes a housing901511, a display portion 901512, speaker portions 901513, operationkeys 901514, a recording medium insert portion 901515, an input means901516, a sensor 901517 (having a function of measuring force,displacement, position, speed, acceleration, angular velocity, rotationnumber, distance, light, liquid, magnetism, temperature, chemicalreaction, sound, time, hardness, electric field, current, voltage,electric power, radial ray, flow rate, humidity, gradient, vibration,smell, or infrared ray), a microphone 901518, an LED lamp 901519, andthe like. The portable game machine shown in FIG. 109A can have variousfunctions such as, but not limited to, a function of reading a programor data stored in the recording medium to display on the displayportion; and a function of sharing information by wireless communicationwith another portable game machine.

FIG. 109B shows a digital camera having a television reception function,which includes a housing 901531, a display portion 901532, operationkeys 901533, a speaker 901534, a shutter button 901535, an imagereceiving portion 901536, an antenna 901537, an input means 901538, asensor 901539 (having a function of measuring force, displacement,position, speed, acceleration, angular velocity, rotation number,distance, light, liquid, magnetism, temperature, chemical reaction,sound, time, hardness, electric field, current, voltage, electric power,radial ray, flow rate, humidity, gradient, vibration, smell, or infraredray), a microphone 901540, an LED lamp 901541, and the like. The digitalcamera having the television reception function shown in FIG. 109B canhave various functions such as, but not limited to, a function ofphotographing a still image and a moving image; a function ofautomatically adjusting the photographed image; a function of obtainingvarious kinds of information from the antenna; a function of storing thephotographed image or the information obtained from the antenna; and afunction of displaying the photographed image or the informationobtained from the antenna on the display portion.

FIG. 110 shows a portable game machine, which includes a housing 901611,a first display portion 901612, a second display portion 901613, speakerportions 901614, operation keys 901615, a recording medium insertportion 901616, an input means 901617, a sensor 901618 (having afunction of measuring force, displacement, position, speed,acceleration, angular velocity, rotation number, distance, light,liquid, magnetism, temperature, chemical reaction, sound, time,hardness, electric field, current, voltage, electric power, radial ray,flow rate, humidity, gradient, vibration, smell, or infrared ray), amicrophone 901619, an LED lamp 901620, and the like. The portable gamemachine shown in FIG. 110 can have various functions such as, but notlimited to, a function of reading a program or data stored in therecording medium to display on the display portion, and a function ofsharing information with another portable game machine by wirelesscommunication.

As shown in FIGS. 101A to 101C, 108A to 108C, 109A and 109B, and 110,the electronic device includes a display portion for displaying somekind of information. Such electronic devices can have a display portionwith improved image quality. Further, the problem of motion blur and theproblem of flicker can be reduced. In particular, an electronic devicehaving a display portion using liquid crystal can include a displayportion with improved viewing angle. An electronic device can include adisplay portion with improved response speed. Since a display portionwith reduced power consumption can be provided, an electronic devicewith reduced power consumption can be obtained. Moreover, since adisplay portion with reduced manufacturing cost can be provided, anelectronic device with reduced manufacturing cost can be obtained.

Next, application examples of the semiconductor device are described.

FIG. 102 shows an example in which the semiconductor device isincorporated in a constructed object. FIG. 102 shows a housing 900810, adisplay portion 900811, a remote control device 900812 which is anoperation portion, a speaker portion 900813, and the like. Thesemiconductor device is incorporated in the constructed object as awall-mounted semiconductor device, which can be provided withoutrequiring a large space.

FIG. 103 shows another example in which the semiconductor device isincorporated in a constructed object. A display panel 900901 isincorporated with a prefabricated bath 900902, and a person who takes abath can view the display panel 900901. The display panel 900901 has afunction of displaying information by an operation by the person whotakes a bath; and a function of being used as an advertisement or anentertainment means.

Note that the semiconductor device can be provided not only for a sidewall of the prefabricated bath 900902 as shown in FIG. 103, but also forvarious places. For example, the semiconductor device can beincorporated with part of a mirror, a bathtub itself, or the like. Atthis time, the shape of the display panel 900901 may be changed inaccordance with the shape of the mirror or the bathtub.

FIG. 104 shows another example in which the semiconductor device isincorporated in a constructed object. A display panel 901002 is bent andattached to a curved surface of a column-shaped object 901001. Note thathere, a utility pole is described as the column-shaped object 901001.

The display panel 901002 shown in FIG. 104 is provided at a positionhigher than a human viewpoint. When the display panels 901002 areprovided in constructed objects which stand together in large numbersoutdoors, such as utility poles, advertisement can be performed to anunspecified number of viewers. Since it is easy for the display panels901002 to display the same images and instantly switch images byexternal control, highly efficient information display and advertisementeffect can be expected. By provision of self-luminous display elements,the display panel 901002 can be useful as a highly visible displaymedium even at night. When the display panel 901002 is provided in theutility pole, a power supply means for the display panel 901002 can beeasily obtained. In an emergency such as disaster, the display panel901002 can also be used as a means to transmit correct information tovictims rapidly.

Note that an example of the display panel 901002 includes a displaypanel in which a switching element such as an organic transistor isprovided over a film-shaped substrate and a display element is driven sothat an image is displayed.

Note that in this embodiment mode, a wall, a column-shaped object, and aprefabricated bath are shown as examples of constructed objects;however, this embodiment mode is not limited thereto, and variousconstructed objects can be provided with the semiconductor device.

Next, examples where the semiconductor device is incorporated with amoving object are described.

FIG. 105 shows an example in which the semiconductor device isincorporated with a car. A display panel 901101 is incorporated with acar body 901102 and can display an operation of the car body orinformation input from inside or outside the car body on demand. Notethat a navigation function may be provided.

The semiconductor device can be provided not only for the car body901102 as shown in FIG. 105, but also for various places. For example,the semiconductor device can be incorporated with a glass window, adoor, a steering wheel, a gear shift, a seat, a rear-view mirror, andthe like. At this time, the shape of the display panel 901101 may bechanged in accordance with the shape of an object to be provided withthe display panel 901101.

FIGS. 106A and 106B show examples where the semiconductor device isincorporated with a train car.

FIG. 106A shows an example in which a display panel 901202 is providedin glass of a door 901201 in a train car, which has an advantagecompared with a conventional advertisement using paper in that laborcost for changing an advertisement is not necessary. Since the displaypanel 901202 can instantly switch images displaying on a display portionby an external signal, images on the display panel can be switched inevery time period when types of passengers on the train are changed, forexample. Thus, a more effective advertisement effect can be expected.

FIG. 106B shows an example in which the display panels 901202 areprovided for a glass window 901203 and a ceiling 901204 as well as theglass of the door 901201 in the train car. In such a manner, thesemiconductor device can be easily provided for a place where asemiconductor device has been difficult to be provided conventionally;thus, an effective advertisement effect can be obtained. Further, thesemiconductor device can instantly switch images displayed on a displayportion by an external signal; thus, cost and time for changing anadvertisement can be reduced, and more flexible advertisement managementand information transmission can be realized.

Note that the semiconductor device can be provided not only for the door901201, the glass window 901203, and the ceiling 901204 as shown inFIGS. 106A and 106B, but also for various places. For example, thesemiconductor device can be incorporated with a strap, a seat, ahandrail, a floor, and the like. At this time, the shape of the displaypanel 901202 may be changed in accordance with the shape of an object tobe provided with the display panel 901202.

FIGS. 107A and 107B show an example in which the semiconductor device isincorporated with a passenger airplane.

FIG. 107A shows the shape of a display panel 901302 provided on aceiling 901301 above a seat of the passenger airplane when the displaypanel 901302 is used. The display panel 901302 is incorporated with theceiling 901301 with a hinge portion 901303, and a passenger can view thedisplay panel 901302 by stretching of the hinge portion 901303. Thedisplay panel 901302 has a function of displaying information by anoperation by the passenger and a function of being used for anadvertisement or an entertainment means. As shown in FIG. 107B, when thehinge portion is bent so that the display panel is stored in the ceiling901301, safety in taking-off and landing can be assured. Note that in anemergency, the display panel can also be used for an informationtransmission means and a guide light by lighting a display element inthe display panel.

Note that the semiconductor device can be provided not only for theceiling 901301 as shown in FIGS. 107A and 107B, but also for variousplaces. For example, the semiconductor device can be incorporated with aseat, a table attached to a seat, an armrest, a window, and the like. Alarge display panel which a plurality of people can view may be providedon a wall of an airframe. At this time, the shape of the display panel901302 may be changed in accordance with the shape of an object to beprovided with the display panel 901302.

Note that in this embodiment mode, bodies of a train car, a car, and anairplane are shown as moving objects; however, the present invention isnot limited thereto, and the semiconductor device can be provided forvarious objects such as a motorcycle, an four-wheel drive car (includinga car, a bus, and the like), a train (including a monorail, a railroadcar, and the like), and a vessel. Since the semiconductor device caninstantly switch images displayed on a display panel in a moving objectby an external signal, the moving object provided with the semiconductordevice can be used as an advertisement display board for an unspecifiednumber of customers, an information display board in disaster, and thelike.

Note that although this embodiment mode is described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in each drawing in this embodiment modewith part of another embodiment mode.

Note that this embodiment mode shows examples of embodying, slightlytransforming, partially modifying, improving, describing in detail, orapplying the contents (or part of the contents) described in anotherembodiment mode, an example of related part thereof, or the like.Therefore, the contents described in another embodiment mode can befreely applied to, combined with, or replaced with this embodiment mode.

This application is based on Japanese Patent Application serial No.2007-133557 filed with Japan Patent Office on May 18, 2007, the entirecontents of which are hereby incorporated by reference.

1. A method for driving a liquid crystal display device, comprising thesteps of: inputting p-th input image data and (p+1)th input image datato the liquid crystal display device at intervals of a period T_(in),wherein p is a positive integer; generating i-th original image data and(i+1)th original image data based on the input image data in a period T,wherein i is a positive integer; generating J number of sub-images basedon the i-th original image data, wherein J is an integer equal to ormore than 3; and displaying the J number of sub-images sequentially inthe period T, wherein at least one of the i-th original image data andthe (i+1)th original image data is in an intermediate state between thep-th input image data and the (p+1)th input image data, at least one ofthe sub-images exhibits a first brightness and other one of thesub-images exhibits a second brightness, and all of the J number ofsub-images are the same images.
 2. The method for driving the liquidcrystal display device according to claim 1, wherein at least one of theJ number of sub-images is a black image.
 3. A method for driving aliquid crystal display device, comprising the steps of: inputting p-thinput image data and (p+1)th input image data to the liquid crystaldisplay device at intervals of a period T_(in), wherein p is a positiveinteger; generating i-th original image data and (i+1)th original imagedata based on the input image data in a period T, wherein i is apositive integer; generating a first sub-image and a second sub-imagebased on the i-th original image data; and displaying the firstsub-image and the second sub-image sequentially in the period T, whereinat least one of the i-th original image data and the (i+1)th originalimage data is in an intermediate state between the p-th input image dataand the (p+1)th input image data, the period T_(in) is more than twicethe period T, and the first sub-image and the second sub-image are thesame images.
 4. The method for driving the liquid crystal display deviceaccording to claim 3, wherein the first sub-image or the secondsub-image is a black image.
 5. A method for driving a liquid crystaldisplay device, comprising the steps of: inputting p-th input image dataand (p+1)th input image data to the liquid crystal display device atintervals of a period T_(in), wherein p is a positive integer; andgenerating i-th original image data and (i+1)th original image databased on the input image data in the period T_(in), wherein i is apositive integer, wherein at least one of the original image data in theperiod T_(in) exhibits a first brightness and other one of the originalimage data in the period T_(in) exhibits a second brightness.
 6. Themethod for driving the liquid crystal display device according to claim5, wherein at least one of the original image data is a black image.